I'm trying to use a command for a map I am creating and I need to test for if an armorstand by the name of Skeletor is wearing a golden helemet on its head. Here's the command I've been using:

/testfor @e[name=Skeletor,Inventory=[{Slot:103,id:golden_helemet}] 

but its says:

The entity UUID provided is in an invalid format.