Nunu/Nasus W bug

I was playing an aram game as nunu against nasus, this bug happened multiple times where nasus would w me and id w to try and negate some of the slow instead i got slower. Unless there is some random factor where if you try and speed up while slowed by nasus (which isnt the case cause ive used ghost to negate the slow before) or something that, i have no idea what happened