Maybe a bug fFiora / Nutiluse

I thinks was I found a bug with the W speel of fiora "Riposte" because this spell tell "if Fiora parries an immobilizing diable, Riposte stun instead of slowing" and with the passif aa " Staggering Blow" of Nautilus when i use this spell nautilus was not stun how this is it a bug or it's like usual

coordialement amiliana summoners of the summoner's rift