Getting APP-SQLAP-97763 error

Hi,

I am trying to create a credit memo in AP to correct an supplier invoice. I did Quantity correction. The original invoice is matched to a PO. As soon as I select invoice for correction I get a Note displayed saying "APP-SQLAP-97763: This PO line is cancelled. You may want to review PO before proceeding with this correction"

I queried the PO neither through the form nor through the database I could see the line is cancelled. (PO has only one line). Then why this note?

Thanks.

Anil