I have tried numerous times to get this setChildIndex( child, 0 ); to work, but I just don’t get why it is not working.

I am trying to call this to change the depth of a movie clip and the compiler keeps spitting out:
ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
at flash.display:: DisplayObjectContainer/setChildIndex()
at game_fla::MainTimeline/frame1()