GAMEPLAY: Karthus invincibilty in ascension

First of all, i only encountered this twice, in ascension. As we are all aware of, when karthus dies in ascension his passive can still become ascended. Doing this will revive karthus. I've noticed two games that somehow, i also become invincible sometimes

1: play ascension, pick karthus 2: get killed by a player, and pick up the ascension buff while undead 3: sometimes, you will be unkillable after being reborn

I have noticed that the two times it happened to me, i had a specific amount of HP. (2k7 with ascension buff) I suspect this happens when you die and revive, while having said amount of health. (to reach the HP amount i believe you need, have scaling health runes, no HP from masteries and buy liandri's torment as ONLY hp item: the bug occurs when you're level 14)