请cg1120等诸位帮助

cg1120给我的裁剪图像程序如下,当运行到bmp.Assign(Image1.Picture.Bitmap);一句时,Image1的图像就没了,这是什么原因呢?
//剪切区域
var
  rgn :HRgn;
  bmp :TBitmap;
begin
  bmp :=TBitmap.Create;
  bmp.Assign(Image1.Picture.Bitmap);
  rgn :=CreateRectRgn(40,40,250,150);
  SelectClipRgn(Image1.Canvas.Handle,rgn);
  Image1.Picture :=nil;
  BitBlt(Image1.Canvas.Handle,40,40,250,150,bmp.Canvas.Handle,0,0,SRCCOPY);
  bmp.Free;
end;