The Challenge offers the following criteria, 
To complete this challenge, add a validation rule which will block the insertion of a contact if the contact is related to an account and has a mailing postal code (which has the API Name MailingPostalCode) different from the account's shipping postal code (which has the API Name ShippingPostalCode).Name the validation rule 'Contact must be in Account ZIP Code'.
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.
The validation rule should ONLY apply to contact records with an associated account. Contact records with no associated parent account can be added with any MailingPostalCode value. (Hint: you can use the ISBLANK function for this check)


I have completed with the following validation rule, which I have tested and works exactly as intended: 

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

I'm not sure why it doesn't complete the challenge. What am I missing?