The current message isTypeError: Argument 4 passed to Symfony\Component\HttpClient\Response\MockResponse::fromRequest() must implement interface Symfony\Contracts\HttpClient\ResponseInterface, instance of Generator given.

I lost some time with this because I was passing a callable that returns a \Generator instead of passing the resulting \Generator directly. We could support that case but I guess with the added exception message, it is clear we don't support it at all.