cann't cast WSession

dear All,

why i cannt cast WSession from Session?

class CommandConvertTolerances extends DefaultUICommandActionListener {
	public void OnCommand() {

		try {

			WSession session = (WSession) pfcSession.GetCurrentSession ();
			
//			WSession wSession = null;	
//						
//			if (wSession instanceof WSession)
//				wSession = (WSession)session;

			Model model = session.GetCurrentModel();
//			WModel wm = (WModel)model;

java.lang.ClassCastException: com.ptc.wfc.Implementation.pfcSession$Session cannot be cast to com.ptc.wfc.wfcSession.WSession

Thanks!

Robert