Sad Amumu Ult

Three times now; once in ranked, once in norms, once in ARAM, I ulted with Amumu just before dying, and the R icon did not go on cooldown. Because it didn't appear on cooldown I tried using it several times and it didn't work obviously. (Leading me to running into the center of the enemy team and dying in the dumbest-looking way imaginable.) This is a dangerous bug that needs attention.