Rabadon's Deathcap doesn't work in ARAM as katarina

When I was playing as katarina in ARAM, I purchased a Rabadon's Deathcap. After some time, I realized why my AP was still so low (lower than usual)? When I pointed my cursor to the item Rabadon's Deathcap, its indicated this: UNIQUE Passive: Increases Ability Power by 30%(0). It's my own problem about updating or it's actually a bug? However, when I pointed to my ally Nunu, his deathcap's passive did work. Thanks.