Admin Intermediate - Formulas and Validations - Create Validation Rules

Hello! 

I'm trying to complete this challenge:  

Create a Validation Rule

Create a validation rule that displays an error message and prevents users from creating or updating a contact for an account if the contact and account have different zip codes. Allow creating and updating contacts that have no associated account.

• Create a validation rule:
  • Rule Name: Contact_must_be_in_Account_ZIP_Code
  • Operator: AND (return true if both conditions are true)
  • Define the two conditions that, combined, will show the error message:
    • The contact is associated with an account id
    • The contact mailing zip code is different than the account shipping zip code 
      Hint: Use the API names (
      MailingPostalCode and Account.ShippingPostalCode) and the <> (Not Equal) operator.
  • Enter an error message for the validation rule

But I keep getting this error message but I'm not sure how to fix my validation rule. I'm selecting through Insert Field Account > "ShippingPostalCode" but that doesn't seem to be correct.  

"

Here is my Error Condition Formula currently that is wrong: 

AND ( 

 Id  <>  ShippingPostalCode  

) 

Any help would be greatly appreciated!