Setting Expense Line Memo Mandatory

Hey All-
I'm working on a script to validate that there is text in the memo field on the expense sublist on a PO (essentially making it mandatory). I wrote a Validate Line function as below. Problem is that it appears it's also firing when a line is added to the item sublist. That's correct, since the validate line function should fire whenever any sublist line is added. I read in the support materials that since the item sublist doesn't have a memo field, it will return null. It also appears to me that when the page initializes it automatically initiates the