Reporting Services Error after Interactive sorting and drill down in pps

Hi 

i have a report  that has a drill down option that shows all workgroups within a specific  parent group and an interactive sorting ass well on both columns  parent group &  workgroup 

This is displayed  in pps combined with a SharePoint

---the problem is that after sorting the parent group column by name   i then press the drill through icon  (+) next to it hopping to see all workgroups related to each parent group  - but instead 

i receive this Reporting Services Error  :

"

• The value for parameter 'SortItem' is not specified. It is either missing from the function call, or it is set to null. (rsMissingParameter) Get Online Help       

"

If I don't press the sorting and just drill through it works fine. the sorting itself also works fine. 

any suggestions ?