Trynd ult failure

In my latest game, I was playing Tryndamere. I was doing a normal thing and 2v1ing their nocturne and amumu. I ulted when I got low and managed to not die for about half of it, until amumu smited me with his skirmishers sabre, and killed me... While i still had 2 seconds of ult left. You can check chat, he was aware that I died during ult. Please check this out, this is awful. UNKILLABLE MEANS UNKILLABLE.