diff --git a/CHANGELOG.md b/CHANGELOG.md
index 040a97cb..665621bf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # next release  
 
+# 5.1.6  
+
+Fixes:  
+- The contentFormatter for ContentType:Json does not correctly format strings (issue #176, thanks @Styn)  
+
 # 5.1.5  
 
 Fixes:  
diff --git a/package-lock.json b/package-lock.json
index 7883f970..f3e7b160 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "swagger-typescript-api",
-  "version": "5.1.5",
+  "version": "5.1.6",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
diff --git a/package.json b/package.json
index b08d7d4e..3b13c6d8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "swagger-typescript-api",
-  "version": "5.1.5",
+  "version": "5.1.6",
   "description": "Create typescript api module from swagger schema",
   "scripts": {
     "cli:json": "node index.js -r -d -p ./swagger-test-cli.json -n swagger-test-cli.ts --extract-request-params --enum-names-as-values",
diff --git a/templates/default/http-client.eta b/templates/default/http-client.eta
index 62b313d6..71b5b0f1 100644
--- a/templates/default/http-client.eta
+++ b/templates/default/http-client.eta
@@ -105,7 +105,7 @@ export class HttpClient<SecurityDataType = unknown> {
     }
 
     private contentFormatters: Record<ContentType, (input: any) => any> = {
-        [ContentType.Json]: (input:any) => input !== null && typeof input === "object" ? JSON.stringify(input) : input,
+        [ContentType.Json]: (input:any) => input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
         [ContentType.FormData]: (input: any) =>
             Object.keys(input || {}).reduce((data, key) => {
             data.append(key, input[key]);
diff --git a/templates/modular/http-client.eta b/templates/modular/http-client.eta
index 3d2a644c..ae92e1cb 100644
--- a/templates/modular/http-client.eta
+++ b/templates/modular/http-client.eta
@@ -105,7 +105,7 @@ export class HttpClient<SecurityDataType = unknown> {
     }
 
     private contentFormatters: Record<ContentType, (input: any) => any> = {
-        [ContentType.Json]: (input:any) => input !== null && typeof input === "object" ? JSON.stringify(input) : input,
+        [ContentType.Json]: (input:any) => input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
         [ContentType.FormData]: (input: any) =>
             Object.keys(input || {}).reduce((data, key) => {
             data.append(key, input[key]);
diff --git a/tests/generated/v2.0/adafruit.ts b/tests/generated/v2.0/adafruit.ts
index 95a3bcd6..9ba7ce8e 100644
--- a/tests/generated/v2.0/adafruit.ts
+++ b/tests/generated/v2.0/adafruit.ts
@@ -256,7 +256,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/another-example.ts b/tests/generated/v2.0/another-example.ts
index 1f24f580..3180bd42 100644
--- a/tests/generated/v2.0/another-example.ts
+++ b/tests/generated/v2.0/another-example.ts
@@ -232,7 +232,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/another-schema.ts b/tests/generated/v2.0/another-schema.ts
index cff35571..ae5e04fd 100644
--- a/tests/generated/v2.0/another-schema.ts
+++ b/tests/generated/v2.0/another-schema.ts
@@ -124,7 +124,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/api-with-examples.ts b/tests/generated/v2.0/api-with-examples.ts
index 3c6a9178..1b2df293 100644
--- a/tests/generated/v2.0/api-with-examples.ts
+++ b/tests/generated/v2.0/api-with-examples.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/authentiq.ts b/tests/generated/v2.0/authentiq.ts
index b367d057..47878481 100644
--- a/tests/generated/v2.0/authentiq.ts
+++ b/tests/generated/v2.0/authentiq.ts
@@ -153,7 +153,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/example1.ts b/tests/generated/v2.0/example1.ts
index f97ff857..065e1dc8 100644
--- a/tests/generated/v2.0/example1.ts
+++ b/tests/generated/v2.0/example1.ts
@@ -128,7 +128,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/file-formdata-example.ts b/tests/generated/v2.0/file-formdata-example.ts
index 23f219cd..5247c131 100644
--- a/tests/generated/v2.0/file-formdata-example.ts
+++ b/tests/generated/v2.0/file-formdata-example.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/furkot-example.ts b/tests/generated/v2.0/furkot-example.ts
index 91647915..b276d514 100644
--- a/tests/generated/v2.0/furkot-example.ts
+++ b/tests/generated/v2.0/furkot-example.ts
@@ -164,7 +164,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/giphy.ts b/tests/generated/v2.0/giphy.ts
index 27463ecf..d7f93da2 100644
--- a/tests/generated/v2.0/giphy.ts
+++ b/tests/generated/v2.0/giphy.ts
@@ -383,7 +383,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/github-swagger.ts b/tests/generated/v2.0/github-swagger.ts
index f0e13caf..2eaafba4 100644
--- a/tests/generated/v2.0/github-swagger.ts
+++ b/tests/generated/v2.0/github-swagger.ts
@@ -1526,7 +1526,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/path-args.ts b/tests/generated/v2.0/path-args.ts
index c59697c4..3feeb8ea 100644
--- a/tests/generated/v2.0/path-args.ts
+++ b/tests/generated/v2.0/path-args.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore-expanded.ts b/tests/generated/v2.0/petstore-expanded.ts
index ec19955a..042eaf51 100644
--- a/tests/generated/v2.0/petstore-expanded.ts
+++ b/tests/generated/v2.0/petstore-expanded.ts
@@ -138,7 +138,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore-minimal.ts b/tests/generated/v2.0/petstore-minimal.ts
index 861e6d29..1285988f 100644
--- a/tests/generated/v2.0/petstore-minimal.ts
+++ b/tests/generated/v2.0/petstore-minimal.ts
@@ -109,7 +109,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore-simple.ts b/tests/generated/v2.0/petstore-simple.ts
index 7bd00391..3d7fe8c0 100644
--- a/tests/generated/v2.0/petstore-simple.ts
+++ b/tests/generated/v2.0/petstore-simple.ts
@@ -124,7 +124,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore-swagger-io.ts b/tests/generated/v2.0/petstore-swagger-io.ts
index 01cc1851..34f6e3cf 100644
--- a/tests/generated/v2.0/petstore-swagger-io.ts
+++ b/tests/generated/v2.0/petstore-swagger-io.ts
@@ -169,7 +169,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore-with-external-docs.ts b/tests/generated/v2.0/petstore-with-external-docs.ts
index a2d9af2c..ba06b108 100644
--- a/tests/generated/v2.0/petstore-with-external-docs.ts
+++ b/tests/generated/v2.0/petstore-with-external-docs.ts
@@ -114,7 +114,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/petstore.ts b/tests/generated/v2.0/petstore.ts
index 448c6d1f..256dd9f5 100644
--- a/tests/generated/v2.0/petstore.ts
+++ b/tests/generated/v2.0/petstore.ts
@@ -116,7 +116,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/query-path-param.ts b/tests/generated/v2.0/query-path-param.ts
index 09ddfe3e..3baab13b 100644
--- a/tests/generated/v2.0/query-path-param.ts
+++ b/tests/generated/v2.0/query-path-param.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v2.0/uber.ts b/tests/generated/v2.0/uber.ts
index c62eaa9e..39023be2 100644
--- a/tests/generated/v2.0/uber.ts
+++ b/tests/generated/v2.0/uber.ts
@@ -196,7 +196,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/additional-properties.ts b/tests/generated/v3.0/additional-properties.ts
index ad8140b3..2735c738 100644
--- a/tests/generated/v3.0/additional-properties.ts
+++ b/tests/generated/v3.0/additional-properties.ts
@@ -108,7 +108,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/additional-properties2.ts b/tests/generated/v3.0/additional-properties2.ts
index 321f9800..d83146b9 100644
--- a/tests/generated/v3.0/additional-properties2.ts
+++ b/tests/generated/v3.0/additional-properties2.ts
@@ -105,7 +105,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/allof-example.ts b/tests/generated/v3.0/allof-example.ts
index e74af7b7..0eb1d07f 100644
--- a/tests/generated/v3.0/allof-example.ts
+++ b/tests/generated/v3.0/allof-example.ts
@@ -109,7 +109,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/anyof-example.ts b/tests/generated/v3.0/anyof-example.ts
index 240c7f9f..47b46ab5 100644
--- a/tests/generated/v3.0/anyof-example.ts
+++ b/tests/generated/v3.0/anyof-example.ts
@@ -111,7 +111,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/api-with-examples.ts b/tests/generated/v3.0/api-with-examples.ts
index 9a4b5ffa..4a90fe90 100644
--- a/tests/generated/v3.0/api-with-examples.ts
+++ b/tests/generated/v3.0/api-with-examples.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/callback-example.ts b/tests/generated/v3.0/callback-example.ts
index 78346133..ff2a0b99 100644
--- a/tests/generated/v3.0/callback-example.ts
+++ b/tests/generated/v3.0/callback-example.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/components-responses.ts b/tests/generated/v3.0/components-responses.ts
index 7b3f176c..bf0340f8 100644
--- a/tests/generated/v3.0/components-responses.ts
+++ b/tests/generated/v3.0/components-responses.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/explode-param-3.0.1.ts b/tests/generated/v3.0/explode-param-3.0.1.ts
index e24b5dd1..4501ea63 100644
--- a/tests/generated/v3.0/explode-param-3.0.1.ts
+++ b/tests/generated/v3.0/explode-param-3.0.1.ts
@@ -117,7 +117,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/full-swagger-scheme.ts b/tests/generated/v3.0/full-swagger-scheme.ts
index cbca8305..cc9a089d 100644
--- a/tests/generated/v3.0/full-swagger-scheme.ts
+++ b/tests/generated/v3.0/full-swagger-scheme.ts
@@ -9045,7 +9045,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/link-example.ts b/tests/generated/v3.0/link-example.ts
index dcf1384a..5ead52b4 100644
--- a/tests/generated/v3.0/link-example.ts
+++ b/tests/generated/v3.0/link-example.ts
@@ -118,7 +118,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/no-definitions-schema.ts b/tests/generated/v3.0/no-definitions-schema.ts
index e0514067..0a1f1f4e 100644
--- a/tests/generated/v3.0/no-definitions-schema.ts
+++ b/tests/generated/v3.0/no-definitions-schema.ts
@@ -114,7 +114,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/nullable-refs.ts b/tests/generated/v3.0/nullable-refs.ts
index 41f49d31..54b0f899 100644
--- a/tests/generated/v3.0/nullable-refs.ts
+++ b/tests/generated/v3.0/nullable-refs.ts
@@ -115,7 +115,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/oneof-example.ts b/tests/generated/v3.0/oneof-example.ts
index cbc13a6f..3a6592f6 100644
--- a/tests/generated/v3.0/oneof-example.ts
+++ b/tests/generated/v3.0/oneof-example.ts
@@ -111,7 +111,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/personal-api-example.ts b/tests/generated/v3.0/personal-api-example.ts
index 23a12d36..1f412358 100644
--- a/tests/generated/v3.0/personal-api-example.ts
+++ b/tests/generated/v3.0/personal-api-example.ts
@@ -302,7 +302,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/petstore-expanded.ts b/tests/generated/v3.0/petstore-expanded.ts
index 14e85e11..0208d69f 100644
--- a/tests/generated/v3.0/petstore-expanded.ts
+++ b/tests/generated/v3.0/petstore-expanded.ts
@@ -114,7 +114,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/petstore.ts b/tests/generated/v3.0/petstore.ts
index 6da484cb..777c14b8 100644
--- a/tests/generated/v3.0/petstore.ts
+++ b/tests/generated/v3.0/petstore.ts
@@ -118,7 +118,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/recursive-schema.ts b/tests/generated/v3.0/recursive-schema.ts
index 2a74589d..42583902 100644
--- a/tests/generated/v3.0/recursive-schema.ts
+++ b/tests/generated/v3.0/recursive-schema.ts
@@ -115,7 +115,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/responses.ts b/tests/generated/v3.0/responses.ts
index fdfc5907..c2e43a9b 100644
--- a/tests/generated/v3.0/responses.ts
+++ b/tests/generated/v3.0/responses.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/swaggerhub-template.ts b/tests/generated/v3.0/swaggerhub-template.ts
index 012f4ad7..ed8ad004 100644
--- a/tests/generated/v3.0/swaggerhub-template.ts
+++ b/tests/generated/v3.0/swaggerhub-template.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/tsoa-odd-types-3.0.2.ts b/tests/generated/v3.0/tsoa-odd-types-3.0.2.ts
index 877eaffe..02984ac1 100644
--- a/tests/generated/v3.0/tsoa-odd-types-3.0.2.ts
+++ b/tests/generated/v3.0/tsoa-odd-types-3.0.2.ts
@@ -222,7 +222,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/up-banking.ts b/tests/generated/v3.0/up-banking.ts
index 1bbacf7b..04394a30 100644
--- a/tests/generated/v3.0/up-banking.ts
+++ b/tests/generated/v3.0/up-banking.ts
@@ -629,7 +629,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/uspto.ts b/tests/generated/v3.0/uspto.ts
index 8b4e36f9..ce4e1b1a 100644
--- a/tests/generated/v3.0/uspto.ts
+++ b/tests/generated/v3.0/uspto.ts
@@ -118,7 +118,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/wrong-enum-subtypes.ts b/tests/generated/v3.0/wrong-enum-subtypes.ts
index 999cb788..1081078e 100644
--- a/tests/generated/v3.0/wrong-enum-subtypes.ts
+++ b/tests/generated/v3.0/wrong-enum-subtypes.ts
@@ -103,7 +103,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/generated/v3.0/wrong-schema-names.ts b/tests/generated/v3.0/wrong-schema-names.ts
index ea9a0596..27ebad78 100644
--- a/tests/generated/v3.0/wrong-schema-names.ts
+++ b/tests/generated/v3.0/wrong-schema-names.ts
@@ -125,7 +125,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/defaultAsSuccess/schema.ts b/tests/spec/defaultAsSuccess/schema.ts
index 233935bc..d616a878 100644
--- a/tests/spec/defaultAsSuccess/schema.ts
+++ b/tests/spec/defaultAsSuccess/schema.ts
@@ -153,7 +153,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/defaultResponse/schema.ts b/tests/spec/defaultResponse/schema.ts
index a852dc27..7a54e15b 100644
--- a/tests/spec/defaultResponse/schema.ts
+++ b/tests/spec/defaultResponse/schema.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/enumNamesAsValues/schema.ts b/tests/spec/enumNamesAsValues/schema.ts
index 35a5194a..8ae614af 100644
--- a/tests/spec/enumNamesAsValues/schema.ts
+++ b/tests/spec/enumNamesAsValues/schema.ts
@@ -299,7 +299,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/extractRequestParams/schema.ts b/tests/spec/extractRequestParams/schema.ts
index 02836a6a..4a8e426d 100644
--- a/tests/spec/extractRequestParams/schema.ts
+++ b/tests/spec/extractRequestParams/schema.ts
@@ -182,7 +182,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/js/schema.js b/tests/spec/js/schema.js
index ae063541..47d30f61 100644
--- a/tests/spec/js/schema.js
+++ b/tests/spec/js/schema.js
@@ -31,7 +31,8 @@ export class HttpClient {
       this.securityData = data;
     };
     this.contentFormatters = {
-      [ContentType.Json]: (input) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+      [ContentType.Json]: (input) =>
+        input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
       [ContentType.FormData]: (input) =>
         Object.keys(input || {}).reduce((data, key) => {
           data.append(key, input[key]);
diff --git a/tests/spec/modular/http-client.ts b/tests/spec/modular/http-client.ts
index d4fc3a77..baec9e24 100644
--- a/tests/spec/modular/http-client.ts
+++ b/tests/spec/modular/http-client.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/moduleNameIndex/schema.ts b/tests/spec/moduleNameIndex/schema.ts
index 5a5bf237..1ed776c8 100644
--- a/tests/spec/moduleNameIndex/schema.ts
+++ b/tests/spec/moduleNameIndex/schema.ts
@@ -229,7 +229,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/responses/schema.ts b/tests/spec/responses/schema.ts
index 3e8759a8..eb32938d 100644
--- a/tests/spec/responses/schema.ts
+++ b/tests/spec/responses/schema.ts
@@ -164,7 +164,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/singleHttpClient/schema.ts b/tests/spec/singleHttpClient/schema.ts
index adc81183..5554edeb 100644
--- a/tests/spec/singleHttpClient/schema.ts
+++ b/tests/spec/singleHttpClient/schema.ts
@@ -101,7 +101,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);
diff --git a/tests/spec/unionEnums/schema.ts b/tests/spec/unionEnums/schema.ts
index a4257b34..96d89240 100644
--- a/tests/spec/unionEnums/schema.ts
+++ b/tests/spec/unionEnums/schema.ts
@@ -111,7 +111,8 @@ export class HttpClient<SecurityDataType = unknown> {
   }
 
   private contentFormatters: Record<ContentType, (input: any) => any> = {
-    [ContentType.Json]: (input: any) => (input !== null && typeof input === "object" ? JSON.stringify(input) : input),
+    [ContentType.Json]: (input: any) =>
+      input !== null && (typeof input === "object" || typeof input === "string") ? JSON.stringify(input) : input,
     [ContentType.FormData]: (input: any) =>
       Object.keys(input || {}).reduce((data, key) => {
         data.append(key, input[key]);