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);            
        }