Making Subordinate fields in Main adress field required

We have been using the field "Main" in the Account record type to store our Account address information.  This field contains several subordinate fields. Previously, these values were updated through an interface from a legacy system.   We are now planning to have the address entered from within CRM-OD and would like to make some of the fields required.  I talked to Oracle support and they suggested I put an expression in the Field Validation for the "Main" field.   I entered a very simple  expression :  [<PrimaryBillToCity>] IS NOT NULL     But,  I am not getting the error message when I save