readyState ist bei IE undefined

function ajax(feld)
{
	this.feld = feld;
	this.http = createObject();
	this.zeilen = 0;
	this.index = 0;
	this.mouse = true;
	this.mousover = false;
	
	this.constructor = ajax;
	this.autosuggest = autosuggest;
	this.autosuggestReply = "";  
};
//--------------------------------------------------------------------------
function createObject() 
{
	var request_type;
	var browser = navigator.appName;
	if(browser == "Microsoft Internet Explorer")  
	{
		request_type = new ActiveXObject("Microsoft.XMLHTTP");  
	}
	else
	{
		request_type = new XMLHttpRequest();
	}
	return request_type;
};
//--------------------------------------------------------------------------
function autosuggest(keycode) 
{	
	if(keycode>46)
	{
		var klasse = this;
		this.autosuggestReply = function()
			{ 	
			  alert(this.readyState);
			  if(this.readyState == 4)
			  {
				  var response = this.responseText;
				  document.getElementById("test").value = response;  
				  e = document.getElementById("results" + klasse.feld);  
				  if(response!="")  
				  {
					  e.innerHTML=response;
					  e.style.display = "block";  
					  klasse.zeilen = document.getElementById("zeilen" + klasse.feld).value;  
					  klasse.index = 0;
				  }
				  else
				  {
					  e.style.display = "none";  
				  }
			  }
			};
		q = document.getElementById('feld' + this.feld).value;  
		nocache = Math.random();
		this.http.open('get', 'lib/search.php?feld='+this.feld+'&q='+q+'&nocache = '+nocache);  
		this.http.onreadystatechange = this.autosuggestReply;
		this.http.send(null);
	}
};