JsonTextParser throw FormatException when the format is good..

I use this code:
JsonTextParser pars = new JsonTextParser(); JsonObject obj = pars.Parse(XXX);
When the XXX equals to:
{ "responseData": { "results":[],"cursor": { "moreResultsUrl":"http://www.google.com/search?oe\u003dutf8\u0026ie\u003dutf8\u0026source\u003duds\u0026start\u003d0\u0026hl\u003den\u0026q\u003d%D7%90%D7%97%D7%9C%D7%94+%D7%91%D7%97%D7%9C%D7%94+%D7%91%D7%92%D7%A8%D7%A0%D7%93+%D7%A7%D7%99%D7%99%D7%9E%D7%9F" } }, "responseDetails": null, "responseStatus": 200 }
And the exception is:
    System.FormatException was unhandled by user code
      Message="Cannot parse a value."
      Source="System.Net.Json"
      StackTrace:
        &n ...