How to create a condition properly in a forms personalization

I want to personalize the AP Invoice Workbench to put a message to the screen when the user enters the same invoice number in one supplier site that already exists in another supplier site. I am doing when_new_record_instance on the inv_sum_folder. I am having trouble with the condition. I have tried a number of things, I created a pl/sql function that returns 1 if I want my message to fire.

I have the following condition entered (select dup_invoice(INV_SUM_FOLDER.INVOICE_NUM,INV_SUM_FOLDER.VENDOR_ID) from dual) = 1 but when trying to validate I get the following error: ORA-00904 INV_SUM_FOLDER.INVOICE_NUM invalid identifier.