Object reference not set to an instance of an object

`try {

                System.IO.MemoryStream stream = new System.IO.MemoryStream();

                _Image.Compress(Bitmap.CompressFormat.Png, 0, stream);
                bitmapData = stream.ToArray();
                }catch (NullReferenceException err) {

                    Console.WriteLine(err.Message);
                }`

Please anyone can tell me why I got the error in subject ? _Image is bitmap object and It's not null. Thanks, Max.