javascript not working ???

anybody got a clue why this javascript want work? i didn't include the script tags and beginning and end of the code here.

it's supposed to parse a tracking url, like http://www.whatever.com?Bob

so that Bob would be printed out.

var URLString = window.location;
var start = URLString.indexOf("?");
var end = URLString.length;
var userName = URLString.substring(start,end);
document.write(userName);

thx in advance