Activity statement showing a random PO number for a payment

Activity Statement issue:

I have a PO reference column (CustPONo) on my activity statement form to show the PO number on all invoice lines.

However on each of the credit (payment received) lines a random PO appears.

As the payment is usually for multiple purchases , the PO reference needs to be blank for this line.

Is there a way around this?

Thanks

Lisa