SAP Message BEA288 - Difference group for item &2/&1 has gaps

Diagnosis
A difference calculation is to be created for billing due list itemSyst-msgv2/syst-msgv1. the system determined a group of billing duelist items for difference calculation. In addition to the currentbilling due list item, this group also contains other not yet billeddue list items.

System Response
The system does not create a difference calculation.

Procedure
Check all predecessors of billing due list item &SYST-MSGV2&/Syst-msgv1 within the difference group. bill any open billing duelist items.