Karthus becoming invincible in Ascension [GAMEPLAY]

I was playing a game of Ascension as Karthus and I went into Xerath and died while being on the Ascension. If you don't know Karthus can still take/contest for Ascension while in his passive. But this time was different, I got the Ascension but now I was invincible.

I've taken Ascension many times as karthus when I die but this is the first time it has happend. It didn't look like something was the main cause of it, it just happend.

However what made me vunerable again was when I was randomly teleported up too top.