Why won't Commander Caius drink?

I've made it through the dampened spirits quest is Skyrim and my objective is to "Attend the tasting ceremony". But Caius just stares at me and won't drink the mead. Is this a bug and is there anyway to fix it without quitting the thieves guild?

I fixed this issue by just waiting 1 hour in game, Commander Caius walked out and Sinmir was drinking instead.

You aren't meant to speak to him, instead speak to Sabjorn after you have poisoned the mead to get the ball rolling. If that fails, I suggest reloading to an earlier save. If that fails and you're on PC, try the following console command: setstage TG03 80.