XMLHttpRequest responseXML is always null

I am calling a asmx web service like this

var xmlhttp;
    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    }
    else {// code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }      
    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4) {
            var data = xmlhttp.responseText;
            var xmlDoc = xmlhttp.responseXML;
        }
    }
    xmlhttp.open("GET", "https://Service/ServiceName.asmx/method?query=data1&count=1",true);
    xmlhttp.setRequestHeader('Content-Type', 'text/xml');
    xmlhttp.send();

even after the readystate being 4, I get responseXML as null and responseText as empty. whereas the url

"https://Service/ServiceName.asmx/method?query=data1&count=1"

works perfectly in the browser.

Please help.