Help! Error " you are trying to read or write into protected memory"

Question

Hi everyone I´m having problems with my code, I´m trying to extract the R,G,B value from an Image, I´m doing this
with the help of System.Runtime.InteropServices.Marshal.Copy, at first I create a pointer to the first
line of the Image, and then I copy all that row data to an array, after that I cover the array and safe the 
R,G,B values into variables. The problem is, once I have finish covering the first row I change my pointer position
adding it an offset but, when I try to execute again this "System.Runtime.InteropServices.Marshal.Copy(realByteAddr, rgbValues, 0, bytes);"
I get the Next Error 

" Access violation was unhandled " you are trying to read or write into  protected memory.
Sometimes this means thatthe memory is damage.

So... any advice??? thank you very much 














































 Here is the code:
static private void RGBextract(Bitmap bmp, out int rojo, out int verde, out int azul)
        {
            int r = 0, g = 0, b = 0;
            int totales = 0;
            
            // Lock the bitmap's bits.  
            System.Drawing.Rectangle rect = new System.Drawing.Rectangle(0, 0, bmp.Width, bmp.Height);
            System.Drawing.Imaging.BitmapData bmpData =
                bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
                bmp.PixelFormat);

            int y = 0;


            while (y < bmp.Height)
            {
                
                // Get the address of the first line.
                int offset = (y * bmpData.Stride);


                int ptr = bmpData.Scan0.ToInt32();

                // Get address of next row 
                IntPtr realByteAddr = new IntPtr(ptr +System.Convert.ToInt32(y * bmpData.Stride));

                // Declare an array to hold the bytes of the bitmap.
                // This code is specific to a bitmap with 24 bits per pixels.
                int bytes = bmp.Width * bmp.Height * 3;
                byte[] rgbValues = new byte[bytes];

                // Copy the RGB values into the array.
                System.Runtime.InteropServices.Marshal.Copy(realByteAddr, rgbValues, 0, bytes); // Copy (source, destination,inicio,longitud)
                // Set every red value to 255. 
                int counter = 0;
                while (counter < bmp.Width)
                {
                    b = b + rgbValues[counter];
                    counter++;
                    g = g + rgbValues[counter];
                    counter++;
                    r = r + rgbValues[counter];
                    counter++;   
                }
                y++;
            }
            totales = bmp.Height * bmp.Width;
            rojo = r/totales;
            verde = g/totales;
            azul = b/totales;
            // Copy the RGB values back to the bitmap
            //System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, realByteAddr, bytes);
            // Unlock the bits.
            bmp.UnlockBits(bmpData);
        }