Mordekaiser creeping death and no assist

I am not sure if anyone else has posted this issue before, but I have noticed I do not get assist from players I have cast creeping death (edit: I meant to say Harvesters of sorrow) on as . If this was intentional I don't understand why spells such as hunters call will provide an assist and not this spell. There are plenty of other spells that can be cast on allies that do no damage to the enemy yet provide assist. I feel this spell can greatly add to a team mates ability to catch up to an evading target and secure the kill, therefore assisting them. If anyone else has had this problem please comment.