Rylai's doesn't affect Karthus' E (and maybe other spells) during passive?

I recently got Karthus in an ARAM game, I was quite fine with it, he's fun enough in ARAM. However, I noticed that when I grabbed a , it worked perfectly fine until I did the whole "run into the enemy team, do a bunch of damage and slow them all, die, do more damage and slow them more" thing... Except the latter didn't happen, because the slow just flat-out stopped working when I was in my passive.