Ajax.getJSON 不触发回调有关问题

$.getJSON("MailTest.ashx", { to: addr, mid: id }, function (json) {
                if (json.result == "OK") {
                    alert("发送成功！请查收");
                } else {
                    alert("发送失败！");
                }
            });

{result:"NO"}
{'result':"NO"}

{"result":"NO"}