Unallocated customer receipts not showing on statement

SOLVED

I have set up a custom statement to show the customer PO on our statements. This has been done by a local join from:

FROM INVOICE_NUMBER to INV_REF_NUMERIC - Parent Outer 

However this works and shows the order number as planned... However when there is an unallocated payment on the account this does not show up. If I delete the field 'INVOICE.CUST_ORDER_NUMBER' the unallocated payment then appears. Can anyone hellp make it so that the payment shows whilst still having this field open?