ActionScript 3.0 :: TypeError: Error #1010 While Accessing A Movieclip From Another Movieclip?

I have got two movieclips on the main timeline. First one is envelope_mc and second is bg_main. In envelope_mc timeline there is a button on 25th frame which plays ahead when clicked. Here's the code for first movieclip:

Code:
on_btn.addEventListener(MouseEvent.CLICK, playAhead);
function playAhead(event:MouseEvent):void
{
play();
}

I have also got some animation on bg_main which I wan't to trigger on 30th frame (inside bg_main timeline) when button is clicked in envelope_mc timeline. Here's the code for second movieclip:

Code:
MovieClip(root).on_btn.addEventListener(MouseEvent.CLICK, playAhead);
function playAhead(event:MouseEvent):void
{
play();
}

Complete error:
TypeError: Error #1010: A term is undefined and has no properties.
at Flashphotographygallerywebsite7_fla::bg_1/frame25()