typeerror error #1010 a term is undefined and has no properties.

I have a student who is working on a short movie with a "Play" button and a progress slider. His code is as follows:

import flash.events.*;

import flash.display.Loader;

import flash.net.URLRequest;

import flash.display.MovieClip;

var loader:Loader = new Loader();

addChild(loader);

loader.mask = mask_mc;

play_mc.buttonMode = true;

play_mc.addEventListener(MouseEvent.CLICK, playClicked);

loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, trackProgress);

function playClicked(event:MouseEvent):void

{

          loader.load(new URLRequest("assets/vid2.swf"));

}

function trackProgress(event:ProgressEvent):void

{

          var perLoaded:Number = Math.round(event.bytesLoaded / event.bytesTotal * 100);

          prog_mc.gotoAndStop(perLoaded);

}

The movie will not play and he receives the following error message:

TypeError: Error #1010: A term is undefined and has no properties.

          at vid2_fla::MainTimeline/frame1()

How can he fix this problem so the movie loads and the progress bar moves.

Thank you.