diff --git a/package.json b/package.json
index c64aa6f..28f5717 100644
--- a/package.json
+++ b/package.json
@@ -33,6 +33,6 @@
         "rollup-plugin-terser": "^7.0.2",
         "rollup-plugin-typescript2": "^0.30.0",
         "ts-jest": "^26.5.4",
-        "typescript": "^4.2.3"
+        "typescript": "^4.8.3"
     }
 }
diff --git a/src/__tests__/Client.test.ts b/src/__tests__/Client.test.ts
index d779893..2c938b8 100644
--- a/src/__tests__/Client.test.ts
+++ b/src/__tests__/Client.test.ts
@@ -55,7 +55,8 @@ describe('data is fetched correctly', () => {
         }
     });
 
-    it('is able to fetch mutations', async () => {
+    // Used service does not provide mutations anymore
+    it.skip('is able to fetch mutations', async () => {
         const result = await client.post(insertUserMutation);
         expect(result).toBeDefined();
 
diff --git a/src/builder/AbstractField.ts b/src/builder/AbstractField.ts
index c4df77a..c108255 100644
--- a/src/builder/AbstractField.ts
+++ b/src/builder/AbstractField.ts
@@ -1,3 +1,4 @@
+import { EmptyObject, StringKeyOf } from '../types';
 import type { HigherKindType, FieldDescendantStore } from './hkt';
 import type { InlineFragment } from './InlineFragment';
 
@@ -121,7 +122,8 @@ export abstract class AbstractField<
     // STRING
     addField<
         NewFieldName extends string,
-        IsArray extends boolean = false
+        IsArray extends boolean = false,
+        NewFieldType = FetchedFieldItemType
     >(
         field: NewFieldName,
         isArray?: IsArray
@@ -130,8 +132,8 @@ export abstract class AbstractField<
         Name,
         FieldReturnType & {
             [k in NewFieldName]: IsArray extends true
-                ? FetchedFieldItemType[]
-                : FetchedFieldItemType
+                ? NewFieldType[]
+                : NewFieldType
         },
         ArrayExpected
     >
@@ -175,13 +177,18 @@ export abstract class AbstractField<
     }
 
     addFieldList<
-        NewField extends string
+        NewField extends string,
+        TypeDefinitions extends Record<string, any> = EmptyObject,
     >(
-        fieldList: readonly NewField[]
+        fieldList: TypeDefinitions extends EmptyObject
+            ? readonly NewField[]
+            : StringKeyOf<TypeDefinitions>[]
     ): HigherKindType<
         this['tag'],
         Name,
-        FieldReturnType & { [K in NewField]: FetchedFieldItemType },
+        FieldReturnType & TypeDefinitions extends EmptyObject 
+            ? FieldReturnType & { [K in NewField]: FetchedFieldItemType }
+            : FieldReturnType & TypeDefinitions,
         ArrayExpected
     > {
         fieldList.forEach(this.addField.bind(this));
diff --git a/src/types/index.ts b/src/types/index.ts
new file mode 100644
index 0000000..9d9109b
--- /dev/null
+++ b/src/types/index.ts
@@ -0,0 +1,4 @@
+export type StringKeyOf<BaseType> = `${Extract<keyof BaseType, string | number>}`;
+
+declare const emptyObjectSymbol: unique symbol;
+export type EmptyObject = {[emptyObjectSymbol]?: never};
\ No newline at end of file
diff --git a/yarn.lock b/yarn.lock
index 0359760..725fa85 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3706,10 +3706,10 @@ typedarray-to-buffer@^3.1.5:
   dependencies:
     is-typedarray "^1.0.0"
 
-typescript@^4.2.3:
-  version "4.2.3"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-4.2.3.tgz#39062d8019912d43726298f09493d598048c1ce3"
-  integrity sha512-qOcYwxaByStAWrBf4x0fibwZvMRG+r4cQoTjbPtUlrWjBHbmCAww1i448U0GJ+3cNNEtebDteo/cHOR3xJ4wEw==
+typescript@^4.8.3:
+  version "4.8.3"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-4.8.3.tgz#d59344522c4bc464a65a730ac695007fdb66dd88"
+  integrity sha512-goMHfm00nWPa8UvR/CPSvykqf6dVV8x/dp0c5mFTMTIu0u0FlGWRioyy7Nn0PGAdHxpJZnuO/ut+PpQ8UiHAig==
 
 union-value@^1.0.0:
   version "1.0.1"