[jQuery] blockUI error

[Jack Killpatrick]

Anyone know what might cause this error, using latest (1.1.1) blockUI
plugin in Firefox 2.x?

From Firebug:

$.blockUI.impl.pageBlockEls has no properties
http://localhost/franklee/js/jquery.blockUI.js?v=1.1.1
Line 320

My code is super simple, based on demo here:
http://malsup.com/jquery/block/#dialog

getSurveyData: function(){
 $.blockUI("<h1>Loading...</h1>" );
  var url = 'myurl.aspx';
  $.getJSON(url, function(data){
     ...some stuff gets done with data...
     $.unblockUI();
  });
}

I've used blockUI for other things in the past, never seen this error. ?

Thanks,
Jack