JSON解析异常

Question

Google.Apis.Json.NewtonsoftJsonSerializer.Instance.Deserialize(Of T)(JsonData)

以上行抛出异常

Newtonsoft.Json.JsonReaderException Error reading string. Unexpected token: StartObject. Path '', line 1, position 1.

当JsonData

时

{"access_token":"ya29.ygAuJKvaVXdJm_QYUgz4oEqN6sjU6W4QwpPsI93RfH-ScW9U1olhgz9S924g7TV5WqDE_Vzfvqx6YQ","token_type":"Bearer","expires_in":3599,"refresh_token":null,"Issued":"11/25/2014 6:59:11 PM"}

但是工作正常wnen JsonData是

{"access_token":null,"token_type":null,"expires_in":null,"refresh_token":null,"Issued":"01/01/0001 12:00:00 AM"}

无法弄清楚原因。有什么指针吗？