[GAMEPLAY] Garen -Ultimate on cooldown, but it says it the chat that it is ready to use

On Garen I was fighting twich, when he was really low I pressed R on him, Garen kinda moved his hands but the ult didint go off, then I noticed the ult icon was in cooldown, but when I Alt-Clicked it, it showed on chat that it was ready to use. Unfortunately I didint tried to cast the ult to see if it was actually on cooldown or not.

1. Let the target get really low in health
2. Use a minion hit to finish him up, but before the minion attack hits the target use the ult on the target
3. Garen will begin to do the hands animation but before he stabs the sword in the ground the minion attack should hit the target
4. The ability will get in cooldown but when you Alt-Click it, it will say in chat that it is ready to use (by the way, in the champions icons it will show the ult indicator full, by other words, ready to use)

It only happened with me once because its really hard to pull it off

https://drive.google.com/open?id=0B4kqlT5mKGRvMGM1N3ZTbm13bk0