Ajax Javascript getElementById error - Emprty String Passed

I am using the following code to get Ajax to update a series of combo boxes based on the input of previous boxes. the javascript calls a PHP database lookup script...

var xmlHttp;

function pop_model(str)
{
xmlHttp=GetXmlHttpObject();
if (xmlHttp==null)
 {
 alert ("Browser does not support HTTP Request");
 return;
 }
var url="ajax/pop_model.php";
url=url+"?q="+str;
url=url+"&sid="+Math.random();
xmlHttp.onreadystatechange=stateChanged("optModel");
xmlHttp.open("GET",url,true);
xmlHttp.send(null);
}

// Check for PHP script completed
function stateChanged(blk)
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
 {
  document.getElementById(blk).innerHTML=xmlHttp.responseText;
 }
}

// Check if appropriate browser is in use for AJAX and determine best method to suit
function GetXmlHttpObject()
{
var xmlHttp=null;
try
 {
 // Firefox, Opera 8.0+, Safari
 xmlHttp=new XMLHttpRequest();
 }
catch (e)
 {
 //Internet Explorer
 try
  {
  xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
  }
 catch (e)
  {
  xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
 }
return xmlHttp;
}

...HTML target is defined via...

                <div id="optModel">
                  <select name="cbModel" class="formbox1" id="cbModel" onChange="pop_variant(document.catsearch.cbModel.value)">
                    <option value="0" selected>All</option>
                  </select></div>

The script runs and the PHP script returns the expected "Select" output but FireBug shows an error of: Empty string passed to getElementById(). and there is no change to the source.