Debit Notes - Invalid Master Account

We've just started using Debit Notes - created when entering Cash
Receipts - and we've hit a problem.

The Cash Receipts Edit List looks fine. The Debit Note is credited
and debited through the Debtor Control Account. But when the Group is
posted, we find that the credit side of the transactions has an
"Invalid Master Account" and hasn't actually been posted. This leaves
us with a balance condition on the Trial Balance and the Debtors
Control Account now differs from the GL Balance.

I've search the Company config and AR Account config looking for a GL
setting that I've missed but haven't found one.

Is anyone else using Debit Notes and are you having the same problem?


TIA,

Nigel.