[Gameplay] Rabadon's Deathcap Passive Always Showing 0% Increased AP

Whenever I buy Rabadon's when I hover over the item with my cursor it always shows that 0 of my ap has been increased. Im pretty sure Rabadons still increases your ap. Its not that big of a deal but it bugs me a lot because I want to know how much ap has been increased. Also this isn't my screenshot its a random screenshot that I found on google images.