Hi All, I'm stuck on the validation rules module. Can anyone help me figure out what I'm missing?

error is: Challenge not yet complete in Mom and Pop

We can't create a contact by itself (without an associated account). Make sure the validation rule applies ONLY when the contact has an associated account.

this is what i have

AND(    NOT(ISBLANK (  Account.Id  )),    ( Account.BillingPostalCode ) <> ( Account.ShippingPostalCode ))