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 6fc46aff26f..5dacb67bf55 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -260,7 +260,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
index ea3594773af..1ab0a775e9c 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index a8137ef03e8..fd8de02dd81 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
index b9cb54736f8..cd6f5def237 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
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 a8137ef03e8..fd8de02dd81 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
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
index a8137ef03e8..fd8de02dd81 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
index a8137ef03e8..fd8de02dd81 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
index a8137ef03e8..fd8de02dd81 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
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 a8137ef03e8..fd8de02dd81 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
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 a8137ef03e8..fd8de02dd81 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
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
index b9cb54736f8..cd6f5def237 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
@@ -269,7 +269,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
index 63afbb449c5..3e4ccfc28f2 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
@@ -265,7 +265,7 @@ export function querystring(params: HTTPQuery, prefix: string = ''): string {
         .join('&');
 }
 
-function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+function querystringSingleKey(key: string, value: string | number | null | undefined | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
     const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
     if (value instanceof Array) {
         const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))