With logo_small_url set to '' (which is valid) you get a broken image at the bottom of the Discobot certificate:
https://try.discourse.org/discobot/certificate.svg?date=Jul+20+1969&user_id=-2
also, because I 
breaking things I can make it 500 with e.g.: https://try.discourse.org/discobot/certificate.svg?date=Jul+00+1969&user_id=-2
