Lissandra Ult Bug

While in a game today, I was using Lissandra. I was playing against a Yasuo, Sion, Draven, Soraka, and Tryandmere. Halfway through the game they were pushing mid, and I was trying to save the tower. I was really weak, so to save myself I used my ultimate. Even though the ult makes me invulnerable, I somehow died during it.