Using ability capsule on Delta Muk results in crash

[Wonder_Mane]

When I use an ability capsule on my Muk I get an error that says

"Script 'PokemonItemEffects' line 1515: NameError occured.

undefined local variable or method 'poke' for nil:NilClass"

If it's of any importance, it's current ability is sap sipper. I don't get a crash if I use it on any other pokemon [in my party at least].