If I remember correctly, this was specifically the case for some users of Bridgy Fed, and the result of this was that trying to look up an actor with an array of urls would cause a 500 error because the url property might or might not return an array. My local fix was just to take the first one in such a case - changing self.ap_actor.get("url") to ap.as_list(self.ap_actor.get("url"))[0]. And then I forgot I'd meant to report the issue and offer my fix back upstream.