No update flags or extensions were set for WBS Element

Hi all.

I am using standart service interface  <n0:ProjectERPWorkBreakdownStructureElementUpdateRequest_sync xmlns:n0="http://sap.com/xi/EA-APPL/SE/Global">

When i am testing this interface through SPROXY i get response:

Object WBS Element XXX could not be changed

No update flags or extensions were set for WBS Element XXX

What flags missing?

I can set action_code in  <WorkBreakdownStructureElement> tag, but it doesn't help.

BR.