A strange issue of Purchase Document Open Interface (POXPDOI)

I tried to use PDOI to update an existing Blanket Purchae Agreement line price.

I inserted a BPA data into PO_HEADERS_INTERFACE and PO_LINES_INTERFACE tables, and submit the concurrent program POXPDOI.

I got error 「Column ITEM_DESCRIPTION should not be NULL.」in PO_INTERFACE_ERRORS table after POXPDOI completed.

But what maked me confused is that whether I populated PO_LINES_INTERFACE.ACTION with "UPDATE" or nothing, the column was populated with "ADD" by POXPDOI , and PO_LINES_INTERFACE.PO_LINE_ID was populated with a new value.

DECLARE

   l_iface_rec         po.po_headers_interface%ROWTYPE;