emguCV中人脸检测中的运行时错误

Question

private void ProcessFrame(object sender, EventArgs arg)
 {


            Image<Bgr, Byte> ImageFrame = capture.QueryFrame();
            if (ImageFrame != null)
            {
                Image<Gray, Byte> grayframe = ImageFrame.Convert<Gray, Byte>();

                          var faces =  grayframe.DetectHaarCascade(haar)[0];

                foreach (var face in faces)
                {
                    ImageFrame.Draw(face.rect, new Bgr(Color.Green), 3);
                }
            }
            CamImageBox.Image = ImageFrame;

        }

我在运行时遇到异常。它说：

NullReferenceException was unhandled

这里：

var faces = grayframe.DetectHaarCascade(haar, 1.4, 4, HAAR_DETECTION_TYPE.DO_CANNY_PRUNING, new Size(25, 25))[0`];