[GAMEPLAY] Cho'gath: R on CD after using, though it did not hit

Steps to reproduce:

1. Use Cho'gath's R on an enemy champion with health low enough that it should execute him
2. While R animation is cast, the champion somehow vanishes (killed by an ally, swallowed by Tahm Kench, got invisible by entering a bush etc.)
3. R is now on cooldown, but the enemy is still alive (edit: if not killed by an ally of course :) ) and lost no health

Today's example: Enemy Riven was low on HP, I used R on her, during animation cast time she vanished in the bushes. She lost no HP and my R was on CD.

Or is that even intended? In that case, I request a change, because it happens to me in like every fifth game or so oO Especially annoying in teamfights.