Darius Ultimate reuse bug

Once, when I was playing with Darius , my team (including me) was** REALLY** fed. In the late game, Cass , and I were hunting down an enemy champion. We've dealt so many dmg in so little time, the enemy's hp bar fell in a split second. I ulted, and Cass used her abilities. Cass officialy got the kill (got the gold, and her score went up), but I was still able to reuse my ulty :S This has helped me because I could ult again, but it's still a bug. Even though Cass got the kill, i could still reuse my Ultimate. This didn't mean much at the time, but it could turn the tides of war at certain situations.