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 4f10c8d1c7d..a8d7af2354c 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -183,7 +183,7 @@ export function exists(json: any, key: string) {
     return value !== null && value !== undefined;
 }
 
-export function querystring(params: HTTPQuery, prefix: string = '') {
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map((key) => {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index 0ed72ba5352..0afb0ba14c0 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -194,7 +194,7 @@ export function exists(json: any, key: string) {
     return value !== null && value !== undefined;
 }
 
-export function querystring(params: HTTPQuery, prefix: string = '') {
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map((key) => {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts
index 0ed72ba5352..0afb0ba14c0 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts
@@ -194,7 +194,7 @@ export function exists(json: any, key: string) {
     return value !== null && value !== undefined;
 }
 
-export function querystring(params: HTTPQuery, prefix: string = '') {
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map((key) => {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
index 0ed72ba5352..0afb0ba14c0 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -194,7 +194,7 @@ export function exists(json: any, key: string) {
     return value !== null && value !== undefined;
 }
 
-export function querystring(params: HTTPQuery, prefix: string = '') {
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map((key) => {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/runtime.ts
index 0ed72ba5352..0afb0ba14c0 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/runtime.ts
@@ -194,7 +194,7 @@ export function exists(json: any, key: string) {
     return value !== null && value !== undefined;
 }
 
-export function querystring(params: HTTPQuery, prefix: string = '') {
+export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map((key) => {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);