Lux combo bug

I played Lux in a coop game and found out that while I was ulting, I couldn't activate my "e" (which has already reached the target location for 3 sec.). I haven't tested yet Lux in a normal game, so I don't know if the bug only occurs in coop games. (Sry for my english.)

P.S.: I'm not sure if it's a bug or if riot just decided to make it that way.