Reference BitmapData within a MovieClip ? [AS3]

programming flash
1

I have a MovieClip that contains a 200x200 png on every frame and nothing else

I am trying to gotoAndStop(); on every frame and then pass the bitmapData within that frame into a function

SS = new SpriteSheet(mc.getChildAt(0),X,Y,0);

I keep getting this error:
TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::Shape@26444089 to flash.display.BitmapData.

Any thoughts?
Thanks.