Cho Gath bug that i found

I play cho gath a lot, and found the bug, when you try to ult the minion and it dies before the animation is finished, the ult goes on cooldown like you didnt kill it, and you dont get the stack. Normally cooldown should be like 30-35 on minions, but it goes on 70+ sec cooldown. Next time if you feast the minion, you will get 2 stacks. This happened to me like 3 or more times.