BitmapData.draw clipping rectangle problem

Ok imagine there is movieclip named my_clip inserted on stage, library movie clip instance with ‘subclip’ linkage and BitmapData “my_bitmap_data”

my_clip.attachMovie(‘subclip’,‘subclip’,1)
my_clip.subclip._x-=my_clip.subclip._width/2
my_clip.subclip._y-=my_clip.subclip._height/2

my_rect = new Rectangle(my_clip.subclip._x,my_clip.subclip._y, my_clip.subclip._width, my_clip.subclip._height)

my_bitmap_data.draw(my_clip,new Matrix(),new ColorTransform(),‘normal’,my_rect);

As you can see I wanna take pixel informations from my_clip (containing subclip which is centered and its pivot point is in the middle) but when I try to insert bitmap data into another clip via attachBitmap it gives only one quarter of the subclip (down-right) which means draw method ignored all graphic under 0 coordinate value. How is this possible?