Problem with onreadystatechange in IE

[elmu]

Hello,

I have a small test application (see the code below) which works fine in Firefox, but not really in IE. As you can see on every value change event I want to display an alert. In case of IE the first 2 changes works fine but after no more alert. If I restart the browser then again 2 times I get the alert message but after nothing.

Any idea what is wrong?

<script language="javascript" type="text/javascript">
<!--
// Get the HTTP Object
function getHTTPObject(){
   if (window.ActiveXObject) return new ActiveXObject("Microsoft.XMLHTTP");
   else if (window.XMLHttpRequest) return new XMLHttpRequest();
   else {
      alert("Your browser does not support AJAX.");
      return null;
   }
}   

// Change the value of the outputText field
function setOutput(){
    if(httpObject.readyState == 4){
		alert("OK");
    }

}

// Implement business logic    
function doWork(){    
    httpObject = getHTTPObject();
    if (httpObject != null) {
        httpObject.open("GET", "category.php?brand="
                        +document.getElementById('brand').value, true);
        httpObject.send(null);
        httpObject.onreadystatechange = setOutput;
    }
}

var httpObject = null;

//-->
</script>

  <form name="testForm" action="">
     <select name="brand" id="brand" onchange="doWork();">
       <option value="Test-1">Test-1</option>
       <option value="Test-2">Test-2</option>
     </select>
  </form>