Why gettng INVALID INVOICE TYPE LOOKUP error with NULL in AP_INVOICES_INTERFACE table?

Been trying to get invoice feed into EBS 12.1.3 using Payables Open Interface, I was hardcoding "STANDARD" into AP_INVOICES_INTERFACE.INVOICE_TYPE_LOOKUP_CODE, but for some incoming invoices getting INVALID INVOICE_TYPE_LOOKUP CODE when APXIIMPT runs, I know from Oracle Payables Documentation, I can leave it NULL in the interface table and Oracle will automatically populate with "STANDARD" or "CREDIT".  Still getting the error?  Any one have explanation and evey better a solution?  Also sometimes getting "NO INVOICE LINES" error as well on same invoices.  On others getting "DUPLICATE INVOICE NUMBER" on other invoices, but I have checked AP_INVOICES_ALL and verified the invoice numbers don't