Karthus Assists

I noticed on Karthus that if you only use your W to slow an enemy and one of your allies kills them you get no assist. Is there a reason why it doesn't? Ashe's E vision does no damage, but if it spots an enemy and dies you get the assist . If you get slowed by Nunu ult, but they escape the ult and gets killed by your ally you get an assist.

Karthus's W gives vision, slows the enemy, and even lowers their magic resist, but no assist. Can this be fixed?