Apparently, it doesn't fail when you enter a name for which it can't find a b.p.o. account. So it knows how to say "beelzebub does not have bpo account" but fails when I put in "bkline" in the GitHub username field. It's tempting to suspect that the 500 error means "yes, I found a b.p.o account for that GitHub user but I just can't say so" but there's no guarantee that this would be a reliable assumption.