NO damage to nexus with twitch

I just finished a game as twitch and decided to ult after both nexus towers were down to destroy the nexus from ranged as there were a lot of minions in the way. i immediately noticed that the nexus appeared to not take any damage whatsoever from my bolts. confused i waited around until my ult ended and i started attacking normally again. at this point the nexus started loosing noticable chunks of hp. i dont believe that the nexus should be immune to twitches ult that seems silly. thank you!