null reference error while using soapclient.js

<script type ="text/javascript">
        function CallSingle() {
            debugger;
            var xml = new XMLHttpRequest();
            var p1 = new SOAPClientParameters();
            var id = document.getElementById("tbId").value;
            var name = document.getElementById("tbName").value;
            var url = "http://localhost:58810/Service1.svc?wsdl";
            xml.open('GET', url, true);
            xml.send();
            SOAPClient.invoke(url, "HelloWorld", p1, true, GetSingleObject_callBack);
        }

        function GetSingleObject_callBack(r) {
            document.getElementById("result1").innerHTML = r;
        }
</script>