diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
index 59fa1f75d57..4b64c995c8b 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
@@ -265,7 +265,7 @@ export class {{classname}} extends runtime.BaseAPI {
             {{/isEnumRef}}
             {{^isEnumRef}}
             {{^withoutRuntimeChecks}}
-            formParams.append('{{baseName}}', new Blob([JSON.stringify({{returnType}}ToJSON(requestParameters['{{paramName}}']))], { type: "application/json", }));
+            formParams.append('{{baseName}}', new Blob([JSON.stringify({{datatype}}ToJSON(requestParameters['{{paramName}}']))], { type: "application/json", }));
             {{/withoutRuntimeChecks}}{{#withoutRuntimeChecks}}
             formParams.append('{{baseName}}', new Blob([JSON.stringify(requestParameters['{{paramName}}'])], { type: "application/json", }));
             {{/withoutRuntimeChecks}}