This message appears when there is a credit entry on one of the invoices for the vendor.  To correct this, in Tool 998: Work with Accounts Payable Vendors, delete the credit entry and enter the information again, so that the credit is the very last entry on the check.  The system will move new entries to the bottom of the invoice.  If there are more than 10 entries, ensure that the second check has a higher debit value than credit value so that the check will not total less than zero.