ActionScript 3.0 :: Imported Tweenlite Class On Main Stage - Error 1009

I am getting a 1009 error using tweenlite. I have placed the COM folder of tweenlite in the same directory as the FLA. On the main stage I have imported the tweenlite class using:
Code:
import com.greensock.*;
import com.greensock.easing.*;

On the main stage I have a MC called mainholdermc. When I click a button called mentions but on the main stage it basically tells mainholdermc to play. This works well. I have another movieclip called vosdes. At the same time that mainholdermc plays I want to use tweenlite to make the aplha of vosdes lower. If I put this actionscript on the main page it works fine:
Code:
mentionsbut.addEventListener(MouseEvent.CLICK, mainholder);
function mainholder(event:MouseEvent):void {
mainholdermc.gotoAndPlay(2);
TweenLite.to(vosdes, 0.4, {alpha:0.1});
}

The problem is that I want to move the tweenlite command to within the the vosdes movieclip. So within the vosdes movieclip I have this written in the actionscript:
Code:
import com.greensock.*;
import com.greensock.easing.*
TweenLite.to(this.vosdes, 0.4, {alpha:0.1});

I found that by using
Code:
this.MC_NAME
it is the equivalent of the _root in AS2.

However this is not working. When I compile the FLA no errors occur until I click on the mentionsbut and I get the following error:
Code:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at com.greensock::TweenLite/init()
at com.greensock::TweenLite/renderTime()
at com.greensock.core::SimpleTimeline/renderTime()
at com.greensock::TweenLite$/updateAll()
[Code] .....