diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
index 942f65369c..0b42e5e862 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -171,11 +171,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/default/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/src/runtime.ts
index 48e54ca6eb..eec2074ab6 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/src/runtime.ts
@@ -182,11 +182,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
index 48e54ca6eb..eec2074ab6 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
@@ -182,11 +182,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/src/runtime.ts
index 48e54ca6eb..eec2074ab6 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/src/runtime.ts
@@ -182,11 +182,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/src/runtime.ts
index 48e54ca6eb..eec2074ab6 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/src/runtime.ts
@@ -182,11 +182,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
index 48e54ca6eb..eec2074ab6 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
@@ -182,11 +182,11 @@ export class Configuration {
         return undefined;
     }
 
-    get headers():  HTTPHeaders {
+    get headers():  HTTPHeaders | undefined {
         return this.configuration.headers;
     }
 
-    get credentials(): RequestCredentials {
+    get credentials(): RequestCredentials | undefined {
         return this.configuration.credentials;
     }
 }