POST请求失败，因为不允许GET请求

我有一个WebMethod在我的代码后面，我通过AJAX调用。该方法在使用GET请求时有效，但我更喜欢使用POST，我也想知道为什么这不起作用和/或我做错了什么。

 $(document).ready(function () {
        $.ajax({
            url: "Default.aspx/HelloWorld",
            method: "POST",
            data: "{}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (data) {
                $("#test1").html(data.d);
            },
            error: function (err) {
                $("#errOutput").text("ERROR: " + err.responseText);
            }
        });
    });

    [WebMethod]
    [ScriptMethod(UseHttpGet=false)]
    public static string HelloWorld() 
    {
        return "Hello World!";
    }

Message:
"An attempt was made to call the method 'u0027HelloWorld'u0027 using a 
GET request, which is not allowed.",
StackTrace:
"at System.Web.Script.Services.RestHandler.GetRawParams(WebServiceMethodData 
methodData, HttpContext context)'r'n 
at System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext 
context, WebServiceMethodData methodData)",
ExceptionType:"System.InvalidOperationException"