Ascension Glitch with Karthus's Passive

Not sure of how to add a replay with this, but today I found a glitch with Karthus's passive. I killed the ascended (NPC Xerath) while in a team fight for it against the enemy team, the whole team was in the circle with the enemy team and I died, so my passive kept me in it. It activated on me while in my passive and I became unhurt-able by any attack. It lasted a good while before it forced me to re-teleport again. After that the match played normally.