WPF - Graphics.CopyFromScreen returns a black image

The following method is taken from a WinForms app. It simply captures the screen, but I needed to modify it to work in a WPF application. When I use it, it returns a black image. The dimensions are correct. I have not got any open DirectX or videos and it wouldn't work even on my desktop.

    public static Bitmap CaptureScreen()
    {
        // Set up a bitmap of the correct size

        Bitmap CapturedImage = new Bitmap((int)SystemParameters.VirtualScreenWidth,
            (int)SystemParameters.VirtualScreenHeight, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

        // Create a graphics object from it
        System.Drawing.Size size = new System.Drawing.Size((int)SystemParameters.VirtualScreenWidth, (int)SystemParameters.VirtualScreenHeight);

        using (Graphics g = Graphics.FromImage(CapturedImage))
        {
            // copy the entire screen to the bitmap
            g.CopyFromScreen((int)SystemParameters.VirtualScreenWidth, (int)SystemParameters.VirtualScreenHeight, 0, 0,
                size, CopyPixelOperation.SourceCopy);
        }
        return CapturedImage;
    }

Can anyone show me the error in my ways?