Gameplay bug, Darius ult not refreshed on kill.

Just played an ARAM game as Darius. My Noxian Guillotine[R] was rank 2. I got a kill on the enemy Warwick with my ult, however it went on cooldown straight away. The buff was displayed above my abilities stating that I could recast Noxian Guillotine, however it was most definitely on cooldown. Unfortunately I didn't grab a screenshot, because I was so perplexed. This was the only incident in the game that it occurred, every other time I got a kill the ult refreshed as intended, so I have no idea how to recreate it.