Amumu Bug

It's probably already been discovered and reported, but I figure it should be posted again in case a rioter didn't see it. Amumu's W is bugged to the point where you can have it toggled on forever, regardless of mana. It constantly causes mana to switch between 0-1. Thought that maybe it was just animation bugged and there was no damage to it, however I let it run dry, walked into minions, and there it goes. Normal damage with no mana. Feel as if this should be fixed ASAP since it could potentially make or break certain situations in games.