onreadystatechange function not being called

<html>
	<head>
	<script>
	function recordScore(str)
	{

	if (str.length==0)
	{

		return;
	}

	xmlHttp=GetXmlHttpObject();
	if (xmlHttp==null)
	{
		alert ("Your browser does not support AJAX!");
		return;
	}

	var url="http://hellworld3.appspot.com/findcountry";
	var params = "screenname="+document.getElementById("screenname1").value+"&score="+document.getElementById("score1").value;
	alert("params: "+params);
	xmlHttp.open("POST",url,true);

	xmlHttp.onreadystatechange = function()
	{
		alert("entered call back function. readstate value is: "+xmlHttpreadyState+". Response Text is: "+xmlHttp.responseText);

	if (xmlHttp.readyState==4)
	{
		document.getElementById("message").innerHTML=xmlHttp.responseText;
	}
	}
	xmlHttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded")
	xmlHttp.send(params);

	//Testing code for synchronous mode
	//alert("Http get status is: "+xmlHttp.status);
	//alert("Http ready state value is: "+xmlHttp.readyState);
	//alert("Http get response text is: "+xmlHttp.responseText);
	//document.getElementById("message").innerHTML=xmlHttp.responseText;
	}


	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;
	}

	</script>


	</head>


	<body>

	<form name="testform">
		Screename:
		<input type="text" id="screenname1" name="screenname">
		<br/>
		Score:
		<input type="text" id="score1" name="score" onchange="recordScore(this.value)">
		<br/>
		<p id="message">test</p>

		<input type="submit" value="Submit">
	</form>

	</body>


	</html>

alert("entered call back function. readstate value is: "+xmlHttpreadyState+". Response Text is: "+xmlHttp.responseText);

xmlHttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xmlHttp.setRequestHeader('Content-length',(params?params.length:0));
xmlHttp.send(params);