Actually please read this post on another thread that talks in more detail about this. You might need to read more of the thread, but that one post contains the key points. You really have to go out of your way to step on this landmine. They tell you in the comment what to do and even if you disobey or misunderstand the intent of those instructions, they still provide you with the safety net to catch the error: that last line before the return that computes the gradients dictionary to return. But you also apparently deleted that. So they were really trying to help you out here to avoid this issue.
I will still file a bug about this, because this whole thing just seems like a waste of time and mental energy to me. What is the real conceptual point they are making here with the test that catches this misbehavior? I’m guessing that your code is simpler and cleaner and what harm is really done by returning an extra gradient?