mvc $.ajax使用属性contentType时500异常

            $.ajax({                type: "POST",                url: "/Ajax/PhoneNumber",                data: {                    number: $("#number").val().substring(0, 7)                },                contentType: "application/json; charset=utf-8",                dataType: "json",                success: function (msg) {                    alert(msg.message);                },                error: function (XMLHttpRequest, textStatus, errorThrown) {                    alert("Status:" + XMLHttpRequest.status + " readyState:" + XMLHttpRequest.readyState + " textStatus:" + textStatus);                }            });

        [HttpPost]        public JsonResult PhoneNumber(string number)        {            string value = BLL.PhoneNumberInfo.Get(number);            return Json(new { message = value }, JsonRequestBehavior.AllowGet);            //return Content(value);                    }