Meat Wagon Disease Cloud as hero ability... how?

[Heresme]

I have the spell disease cloud made into a hero ability, specifically meat wagon. So at the point of the attack, it creates a unit (disease cloud).

It has four levels, each creating a stronger disease cloud. Each disease cloud has the disease cloud (plague ward) ability of varying strengths.

Unfortunately it doesn't seem to work. The disease cloud sits there for a certain amount of time, but units walking over it do not get diseased. Am I doing something wrong?

Disease Cloud (meat wagon) (hero ability) creates disease cloud (unit) which has disease cloud (plague ward) (unit ability).