Possible Bug with Kalista's Rend

Hi, just reporting a bug I found while playing Kalista (Sorry I didn't catch this on the PBE, didn't have the scenario happen there.)

When Kalista's Rend, her E, is activated and kills a target, it's suppose to not go on cooldown. However, I used Rend on Zilean right before he placed his revive spell onto himself, and "killed" him with it. The spell went on cooldown, however, and Zilean revived as normal. I'm really not sure if this is intended or not, but just in case it isn't, I'm sending in a quick bug report.