Creating preloader and progress bar -Tutorial

Hi,

I studied the above mentioned tutorial.
I used this code.

function SetProgress(value:Number)
{

progress.width = value*base.width;

}
preloader.SetProgress(.5);

When I experimented it, got an error like this.

Access of undefined property base.

I gave all the movieclip names correctly.
Can anyone tell me what is the problem?