unsafe code i modified isn't working quite right.. (C#)

[David24]

my best attempt to get the following code to store 2 ints representing the average location of x and y coordinates is throwing NullExeption errors. this is unsafe C# code by the way. i'm a novice so hopefully you will notice some obvious, glaring, simple solution for this. it's very important to me that i get this working. it would be a better motion detector than the one i currently use on my motion tracking turret. (although the old detector does the job well enough, it uses a blobcounting (from the Aforge library) algorithm that just doesn't act right)

else
            {
                // calculate motion without suppressing noise
                byte* motion = (byte*)currentFrame.ToPointer();

                for (int i = 0; i < frameSize; i++, motion++)
                {
                    pixelsChanged += (*motion & 1);//compares the last bit of the motion value if it was set to one.
                }
            }

            // highlight motion regions
            if (highlightMotionRegions)
            {
                byte* src = (byte*)imageData.Scan0.ToPointer();
                byte* motion = (byte*)currentFrame.ToPointer();
                int srcOffset = imageData.Stride - width * 3;

                // shift to the red channel
                src += 2;

                int coordinateCount = 0;

                for (int y = 0; y < height; y++) // do this 480 times
                {
                    for (int x = 0; x < width; x++, motion++, src += 3)//do this 640 times
                    {
                        *src |= *motion; //the address of source or motion = the address of source

                        //i added this. if this is a motion pixel, store it's coordinates
                        if (*motion > 0)
                        {
                            //x is guide to what pixel is being tested so store it as a coordinate
                            xCoord[coordinateCount] = x;
                            //the same with y
                            yCoord[coordinateCount] = y;
                            //increase the array number so the next values aren't written over the first
                            coordinateCount++;                           
                        }

                    }
                   src += srcOffset;//moves the pointer to the next adress where to get the next data.
                }

                //i added this to get the average value of the ints in the x and y arrays
                int sumX = 0;
                for (int i = 0; i < coordinateCount; i++)
                {
                    sumX += xCoord[i];
                }
                int targetX = sumX / coordinateCount;

                int sumY = 0;
                for (int i = 0; i < coordinateCount; i++)
                {
                    sumY += yCoord[i];
                }
                int targetY = sumY / coordinateCount;
            }

            // unlock source image
            image.UnlockBits(imageData);

[kenobewan]

Would be easier if you paste the error and give the line number. Thanks.