Error while save image after watershed the image

void Readimage (OpenFileDialog openFileDialog)
 {
    //Read input image
     var image = new Image<Bgr, Byte>(openFileDialog.FileName);
     //Get the binary image
     Image<Gray, Byte> binary = image.Convert<Gray, Byte>().ThresholdBinaryInv(new Gray(140), new Gray(255));
        var closeElement = new StructuringElementEx(5, 5, 2, 2,Emgu.CV.CvEnum.CV_ELEMENT_SHAPE.CV_SHAPE_ELLIPSE);
        binary = binary.MorphologyEx(closeElement, Emgu.CV.CvEnum.CV_MORPH_OP.CV_MOP_CLOSE, 1);

        //Eliminate noise and smaller objects
        Image<Gray, Byte> foreground = binary.Erode(6);

        //Identify image pixels without objects
        Image<Gray, Byte> background = binary.Dilate(6);
        background._ThresholdBinaryInv(new Gray(1), new Gray(128));

        //Create markers image
        Image<Gray, Byte> markers = background + foreground;
        //Create watershed segmentation object
        WatershedSegmenter watershedSegmenter = new WatershedSegmenter();
        //Set markers and process
        watershedSegmenter.SetMakers(markers);
        Image<Gray, Int32> boundaryImage = watershedSegmenter.Process(image);
        string appPath = Path.GetDirectoryName(Application.ExecutablePath) + @"\ProImages\";
        boundaryImage.ToBitmap().Save(appPath);}