Ascension Bug with Karthus

I don't think there's a way I can tag this as its just saved as two replays on my client, but i was playing ascension and came to a point where there was a karthus who was about to ascend, but i killed him, and while he killed me in his passive HE ascended (whilst in passive) and from that point on because he was in passive the whole time, he couldnt take any damage!!!