I'm on the final part of the Formulas & Validations Badge, and have confirmed that my validation rule is working correctly. When I test my validation formula for the assignment, it works fine. Here's the rule I'm using.

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

The assignment is to create a contact with a MailingPostalCode that has an account and does not match the associated Account ShippingPostalCode should return with a validation error and not be inserted. It also should apply to contact records with an associated account. Contact records with no associated parent account can be added with any MailingPostalCode value. 

I believe this is setup as needed, but the error its throwing is my validation rule saying that an insert failed because of the rule? Any ideas on how to fix this?