Problem with player heads

Hello, i have a problem with the player heads.
Tell me if i'm right.
I created the item head with the value and i Named it ExpHead then i verify it doing /mm items give aldo1501 ExpHead.
Then I create the mob with this config
Skills:
- equip{i=ExpHead:4} ~onSpawn
When i spawn the mob with /mm mobs spawn Exp 1 (is the name of the mob) the mob spawn without the head
WHYYYYYY?
I'm becoming mad.
(sorry for my english i'm italian)