Javascript Error: Reference to undefined variable: No such variable 'XMLHttpRequest'

In Opera 7.23 I'm getting the following error on my Ajax code:

message: Reference to undefined variable: No such variable 'XMLHttpRequest'

My create XMLHttpRequest code:

function createRequestObject()
{
    var request_o; //declare the variable to hold the object.
    var browser = navigator.appName; //find the browser name
    if (browser == "Microsoft Internet Explorer")
       {
         /* Create the object using MSIE's method */
         request_o = new ActiveXObject("Microsoft.XMLHTTP");
       }
    else
       {
         /* Create the object using other browser's method */
         request_o = new XMLHttpRequest();
       }
    return request_o; //return the object
}

var http = createRequestObject();

Can anyone explain why I'm getting this JS error?