flutter firebase cloud function json exception

i am testing firebase_cloud function with flutter and in index.js file i wrote like below

export const helloWorld = functions.https.onCall((data, context) => {
  return {
    message: "Hello World"
  };
});

and i call the function like bellow

  var c = await aa.getHttpsCallable(functionName: 'helloWorld');
  var cc = await c.call();

but when i run this code

Exception has occurred.
PlatformException (PlatformException(functionsError, Cloud function failed with exception., {message: Response is not valid JSON object., details: null, code: INTERNAL}))

flutter say like this
in javasscript it return json but why error occur?