Amumu ult Bug

This has got me a couple times; after spamming my ult because i have terrible ping, if i keep hitting it after its on cd, after it comes off cd I cant use it again. I've lost three games in ranked because of it just throwin that out there.

EDIT- so i tried recreating it in a custom game following my normal build path and couldnt recreate it. I'll have to just record my games and the next time i catch it ill post it here. In the mean time, its there just be aware.