Request reviewers endpoint always returns a happy 201... it shouldn't

Notice in the docs that the endpoint [POST] /repos/{owner}/{repo}/pulls/{pull_number}/requested_reviewers is supposed to return a 422 if a user is not a valid contributor.

I’m not getting that result. I passed a wacky user id that can’t possibly be a contributor on our repo, and still got a 201.

See below:

Is this a bug? Am I using the API wrong?