GetAbilityCost returns nil for power type

[LadyTiny]

I've tried using GetAbilityCost, GetAbilityCostOverTime, and GetSlotAbilityCost. All three of them return the the first argument, for example 3500 for cost, but the second argument is always nil.

local cost, mechanic = GetAbilityCost(abilityId)

Mechanic is always nil. Is there any other way to find out whether an ability is a magicka or stamina or health costing ability?