Why do an 'internal error' is returned for a cloud function call by HTTPScallable?

I am calling a cloud function using httpscallable, but it gives me an error:[firebase_functions/internal] internal

Code snippet as follows:

customerPortal() async {    debugPrint('customer portal pressed');    HttpsCallable callable = FirebaseFunctions.instance        .httpsCallable('ext-firestore-stripe-payments-createPortalLink');    try {      HttpsCallableResult result =          await callable.call({'returnUrl': 'https://cancel.com'});      debugPrint(result.data);    } on FirebaseFunctionsException catch (error) {      debugPrint(error.toString());    }  }

Why do I get this error? Any fix?