Error on renderworkspace

        private void DrawObject()
        {
            //Create a rectangle object
            Aurigma.GraphicsMill.WinControls.RectangleVObject rectangle = new
                Aurigma.GraphicsMill.WinControls.RectangleVObject(100, 100, 200, 200);

            //Modify stroke and fill parameters
            rectangle.Pen = new System.Drawing.Pen(System.Drawing.Color.Black, 5F);
            rectangle.Brush = new System.Drawing.SolidBrush(System.Drawing.Color.Black);

            //Add the rectangle to a layer
            vObj.Layers[0].VObjects.Clear();
            vObj.Layers[0].VObjects.Add(rectangle);

            //Render the rectangle to a bitmap            
            Aurigma.GraphicsMill.Bitmap renderedVObjects = new Aurigma.GraphicsMill.Bitmap();
            [u]renderedVObjects = vObj.RenderWorkspace();[/u]            
            ////Merge the two bitmaps
            renderedVObjects.Draw(bitmapViewerMainView.Bitmap, 0, 0, bitmapViewerMainView.Bitmap.Width,
                bitmapViewerMainView.Bitmap.Height, Aurigma.GraphicsMill.Transforms.CombineMode.Alpha,
                1f, Aurigma.GraphicsMill.Transforms.InterpolationMode.HighSpeed);

            ////Clear the rubberband workspace
            vObj.Layers[0].VObjects.Clear();        
        }