jQuery returns null for element height? why?

Question

hey guys,

$(document).ready(function(){

  function getDocumentHeight()
  {
     // What's the page height?
     var height = $('#mainbody').height();
     alert(height);
   ...
 getDocumentHeight();

jQuery keeps alerting "null" for the height of my #mainbody div. i don't get why, it should at least be 500px or so.

thank you.