Viktor E bug

I was playing Viktor just for fun to test him out, and as I used my E on Karthus, instead of dealing damage, for whatever reason, it removed the visuals on my health and mana pool on the bottom bar. First time I've seen this, also first time I've played Viktor. I have zero idea what caused it, but you guys should totally look into it.

(EDIT) Just did it again with 2 other champs with a friend, seems to be a karthus only interaction. I've done it twice now to him. This needs to be fixed ASAP. It's not game breaking, but really really annoying.