ActionScript 3.0 :: Using A Function To Set X Position?

I've declared a global variable called thumbSpace.I have an xmlLoaded function that loads in thumbnails from an xml file using a loop.
I have an event listener call the thumbLoaded function when the thumbnail is done loading.The thumbLoaded function returns the width of each thumbnail.

Code:
function thumbLoaded(event:Event):Number {
return event.target.width;
}

Inside of the xmlLoaded function I have the following lines of code:

Code:
thumbSpace += thumbLoaded();

I keep getting the "error message 1136: Incorrect number of arguments. Expected 1."� I'm trying to use the thumbLoaded function to set the X position of my thumbnails.