Tyrndameres ult can be silenced

I've seen people complain before that tyrn having an ult that cant be silenced is terrible, but recently in an aram map, my ult was not only silenced by cho'gath's w, but it was glitched in that it didn't provide immunity to damage for the rest of the match. In other words, cho'gaths w made my ult useless for the remainder of the match.