diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index de106bae2cdf3..d47bde43334c9 100755
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -6,7 +6,7 @@
 namespace ts {
     const ignoreDiagnosticCommentRegEx = /(^\s*$)|(^\s*\/\/\/?\s*(@ts-ignore)?)/;
 
-    export function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName = "tsconfig.json"): string {
+    export function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName = "tsconfig.json"): string | undefined {
         return forEachAncestorDirectory(searchPath, ancestor => {
             const fileName = combinePaths(ancestor, configName);
             return fileExists(fileName) ? fileName : undefined;
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 464e70671f4bd..4fe717206d23e 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -3775,7 +3775,7 @@ declare namespace ts {
     }
 }
 declare namespace ts {
-    function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string;
+    function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string | undefined;
     function resolveTripleslashReference(moduleName: string, containingFile: string): string;
     function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost;
     function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index bc7115a089a89..48d07de68715b 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -3722,7 +3722,7 @@ declare namespace ts {
     }
 }
 declare namespace ts {
-    function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string;
+    function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName?: string): string | undefined;
     function resolveTripleslashReference(moduleName: string, containingFile: string): string;
     function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost;
     function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[];