Kleptomancy Gameplay

I just played as Vel'koz with Kleptomancy against a Yasuo. Obviously, Yasuos windwalls blocks projectiles. Yet, when I cast a spell and he windwalls, if I auto attack him afterwards, the auto will be blocked by the windwall but Kleptomancy is still proc'd.

To replicate: throw a spell at yasuo. Have him windwall, and then auto attack him and make sure it hits the windwall. You will proc Kleptomancy. At least, it did on Vel'Koz