unsafe code i modified isn't working quite right..

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);