jquery json context.request System.NullReferenceException: 未将对象引用设置到对象的实例

var data = context.Request;
var sr = new StreamReader(data.InputStream);
var stream = sr.ReadToEnd();
var javaScriptSerializer = new JavaScriptSerializer();
var PostedData = javaScriptSerializer.Deserialize<RequestDataJSON>(stream);

string userid = PostedData.idd;
string oldpwd = PostedData.newpwd;

string newpwd = PostedData.oldpwd;