[GAMEPLAY] Can't Q nexus with garen (can't possibly hit it when activated)

Playing Garen (Dreadknight skin don't know if it's only this skin or not) and was finishing the game. Nexus was 5000 hp auto hit then wanted to Q to reset auto and maybe add more damage. And Garen was chilling around during the whole Q. Then Q disapeared and was like "wth" , tried it again and every time it's the same problem. You can't Q the nexus.