Getting error message when creating bitmap image.   

I am trying to plot my pushpins manually, but when I go to create the bitmap, 
I get an error message.  Has anyone gotten this message before?

code:
            MapImage[] mapImages = render.GetMap(mapSpec);
            MapImage mi = new MapImage();
            mi = mapImages[0];

            Bitmap newBitmap = new Bitmap(800, 600);
            Graphics graphics = Graphics.FromImage(newBitmap);
            Bitmap mapBitmap = new Bitmap(new 
System.IO.MemoryStream(mi.MimeData.Bits));

I get error message when the bitmap line that says:
Object reference not set to an instance of an object.

date: Mon, 22 Oct 2007 06:16:03 -0700   author:   Amy