Karthus/Rylais

Just a few minutes ago I was in an ARAM game playing as Karthus, I had the item "Rylai's Crystal Scepter" which applies a slow to any spell damage you deal, I was using my "E" ability "Defile" which deals AoE damage as long as it is toggled on and the caster has the mana to sustain. I chased the enemy Sona down as she could not escape the slow of my spell, though she did kill me before I could kill her. I noticed, however, that when I died my "Defile" failed to proc the crystal scepter passive though sona stood within its range for a further 2 seconds. It appears that if a champion is within Defile's range and it is active when Karthus dies, it fails to proc on hit spells? Or maybe just rylais? Please enlighten me.