How to sort picklist (lookup field) descending

How to sort picklist (lookup field) descending

    I have this problem: I have a form "two" which is linked to form "one" through "record number" lookup field. I want always to enter data into last record of form "two". That's why I would like to have the last record of the lookup field on the most top.
      I wrote the following script
   
record number
        (
            displayname  =  "record number"
            type  =  picklist
            values  =  safetyreport.safetyreportid
            sortorder  =  descending
            width  =  206px
        )
If the sort order value is "ascending" it's working but if the value is "descending" I get an error message. Where is the problem?






Thanks anyone who can help.