We created a contact with a 'MailingPostalCode' that's different from the associated account's 'ShippingPostalCode'. We expected the validation rule to prevent that, but it didn't. Make sure the validation rule returns true and prevents creating a contact

I continue to get this error in the Create Validation Rules Trail.

I have the validation rule set up in Contact 

This is my formula 

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

I have an error message 

What else am I missing?????