According to HELP "You cannot change the WindowType setting once the window has been displayed.". Strange but sometimes some of my forms are displayed in Init Event (even not complete all the code in it). I can't find the reason yet. Try

LPARAMETERS lpnDataSessionId, lpnWindowtype, lpnApr_id
WITH thisform
	IF VARTYPE(m.lpnWindowtype)="N" AND ;
           INLIST(m.lpnWindowtype,0,1)  AND ;
           .WindowType # m.lpnWindowtype ;
           .Hide()
           .WindowType = m.lpnWindowtype
           .Show()
	ENDIF
ENDWITH