Diagnosis
You cannot save internal key 0 in the rental unit dialog as 0 is theinitial value of the field.

System Response
Use another internal key to enter Customizing data for a floor that youwant to maintain in the rental unit dialog.

Procedure
You should keep a dummy entry for 0 so that rental units where no floordata has been entered are still displayed with the 'General rentalunits" matchcode search.