Zilean Ultimate didn't work

Hey! Today i played a game on botlane, me as Kalista and my Teammate as Blitzcrank against Zilean/Varus. I think we all were just level 6, and we got into a fight in which Zilean ulted Varus, and as Varus was low, i activated rend (Zil Ult was still active), and instead of him being revived, it said "enemy slain"(the usual thing), and Varus was not revived. You could even see the golden balls when he was laying dead for a while. Yea, thats basically it.