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 741a1ba5545..4f10c8d1c7d 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -189,12 +189,12 @@ export function querystring(params: HTTPQuery, prefix: string = '') {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
             const value = params[key];
             if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(singleValue))
+                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
                     .join(`&${encodeURIComponent(fullKey)}=`);
                 return `${encodeURIComponent(fullKey)}=${multiValue}`;
             }
             if (value instanceof Object) {
-                return querystring(value, fullKey);
+                return querystring(value as HTTPQuery, fullKey);
             }
             return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
         })
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index 593648d3776..0ed72ba5352 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -200,12 +200,12 @@ export function querystring(params: HTTPQuery, prefix: string = '') {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
             const value = params[key];
             if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(singleValue))
+                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
                     .join(`&${encodeURIComponent(fullKey)}=`);
                 return `${encodeURIComponent(fullKey)}=${multiValue}`;
             }
             if (value instanceof Object) {
-                return querystring(value, fullKey);
+                return querystring(value as HTTPQuery, fullKey);
             }
             return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
         })
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 593648d3776..0ed72ba5352 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/runtime.ts
@@ -200,12 +200,12 @@ export function querystring(params: HTTPQuery, prefix: string = '') {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
             const value = params[key];
             if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(singleValue))
+                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
                     .join(`&${encodeURIComponent(fullKey)}=`);
                 return `${encodeURIComponent(fullKey)}=${multiValue}`;
             }
             if (value instanceof Object) {
-                return querystring(value, fullKey);
+                return querystring(value as HTTPQuery, fullKey);
             }
             return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
         })
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 593648d3776..0ed72ba5352 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -200,12 +200,12 @@ export function querystring(params: HTTPQuery, prefix: string = '') {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
             const value = params[key];
             if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(singleValue))
+                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
                     .join(`&${encodeURIComponent(fullKey)}=`);
                 return `${encodeURIComponent(fullKey)}=${multiValue}`;
             }
             if (value instanceof Object) {
-                return querystring(value, fullKey);
+                return querystring(value as HTTPQuery, fullKey);
             }
             return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
         })
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 593648d3776..0ed72ba5352 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
@@ -200,12 +200,12 @@ export function querystring(params: HTTPQuery, prefix: string = '') {
             const fullKey = prefix + (prefix.length ? `[${key}]` : key);
             const value = params[key];
             if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(singleValue))
+                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
                     .join(`&${encodeURIComponent(fullKey)}=`);
                 return `${encodeURIComponent(fullKey)}=${multiValue}`;
             }
             if (value instanceof Object) {
-                return querystring(value, fullKey);
+                return querystring(value as HTTPQuery, fullKey);
             }
             return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
         })