Why does jQuery return null for the height of the div?

$(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.