create new element and add

error "MicroStationDGN._ModelReference.AddElement(MicroStationDGN._Element)"

i don't understand this error, please help me

     MicroStationDGN.Application micro = new MicroStationDGN.Application();

            designFile = micro.ActiveDesignFile;
            Point3d start_point = new Point3d();
            Point3d end_point = new Point3d();

            start_point.X = 59.0;
            start_point.Y = 200.0;

            end_point.X = 259.0;
            end_point.Y = 200.0;
            designFile.Models[2].Activate();
            micro.ActiveModelReference.Activate();

            MicroStationDGN.LineElement testLine = micro.CreateLineElement2(null, ref start_point, ref end_point);

            micro.ActiveModelReference.AddElement(testLine);
            testLine.Redraw();