Go to https://phabricator.wikimedia.org/conduit/method/policy.create/ and enter "ZZZZ" in the "objectType" field. I'd expect a proper error, I get htmlspecialchars() expects parameter 1 to be string, object given instead.

Basically that's a TypeError, because PhabricatorConduitAPIController::renderAPIValue(Aphront404Response) just won't work.