使用jquery ajax jsonp调用Web服务:错误

我正试图使用ajax调用Web服务，我得到了一个内部500错误。你能告诉我不确定我做错了什么吗，我可以毫无问题地调用Web方法。

JQUERY AJAX CALL
<script type="text/javascript">
function LoginVailid() {
    $.ajax({
        url: "http://localhost:49301/AppService.asmx/LoggonAuthentication",
        contentType: "application/json; charset=utf-8",
        data: "{}",
        dataType: "jsonp",
        jsonp: "callback",
        crossDomain: true,
        success: function (json) {
            alert(json.d);
        },
        error: function () {
            alert("Hit error fn!");
        }
    });
}
</script> 

WEBSERVICE方法

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
[System.Web.Script.Services.ScriptService]
public class Service1 : System.Web.Services.WebService
{
    [WebMethod]
    [ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
    public string LoggonAuthentication()
    {
        return "Hello World";
    }
}

[WebMethod]
[ScriptMethod(UseHttpGet = true, XmlSerializeString=false, ResponseFormat = ResponseFormat.Json)]
public string LoggonAuthentication(string callback)
{
    return  callback + "({message: 'Hello World'})";
}