NON-PO invoices is processed as PO Invoices in WFR

Hello Experts,

we are using the WFR 11.1.1.9.0. we have processed one non-po invoices and it has been processed as PO Invoices without error as it's having the below scenario

1) It took the some value from Invoice document  and processed as PO Number

2) This value is available in PO Database Table.

please advise how we can stop this behavior, so it won't process NON-PO Invoices as PO Invoices.

Thanks

Sanjeev