Invalid Payment Method When NULL on Invoice Import for Certain OUs Only

Anyone know why the AP import of invoices via the Payables Open Invoice Interface rejects invoices for some operating units (OUs) when the Payment Method Code column is NULL, but does not reject invoices for other OUs when the same column is NULL?
Is there an optional default Payment Method value for an OU?

We've been importing lots of invoices without any Payment Method Code value successfully into a few OUs.  But now that we're starting to import into other OUs, we're starting to see Invalid Payment Method for certain OUs.

In all cases, our imported invoices (processed or rejected) have a NULL value in the interface's Payment Method column.  Payment Method is also NULL on all of our suppliers and supplier sites.