Deserialization of json objects in Restsharp

I have the following code that uses RestSharp to call a web service. The code runs with no errors, but when I look at the response.Data I see that the Result object was not deserialized, and still contains the default values.

<Serializable()> <DataContract()> _
Private Class Result
  <DataMember(IsRequired:=False)> _
  Public responseCode As Long
  <DataMember(IsRequired:=False)> _
  Public responseMessage As String
  Friend Const RESULT_OK As Long = 2000
End Class

Private Function Login(ByVal user As String, ByVal password As String) As  RestResponseCookie
  Dim req As New RestRequest("authenticationService/authentication/signIn", Method.POST)
  req.RequestFormat = DataFormat.Json
  Dim input As New SigninInput
  input.requestParameters.username = user
  input.requestParameters.password = password
  req.AddBody(input)

  Dim response As RestResponse(Of Result) = getRestClient().Execute(Of Result)(req)
  If response.StatusCode = HttpStatusCode.OK Then
    Dim res As Result = response.Data
  End If
  Return Nothing
End Function

Now, if I change the code to the following:

  Dim response As RestResponse = getRestClient().Execute(req)
  If response.StatusCode = HttpStatusCode.OK Then
    Dim des As New Json.DataContractJsonSerializer(GetType(Result))
    Dim res As Result = CType(des.ReadObject(New IO.MemoryStream(response.RawBytes)), Result)
  End If

Deserialization runs just fine. I tried to debug the result from the non-working version of the code, and the content appears to be fine. This is the Content I get: "{"responseMessage":"Success: Your request was successfully completed.","responseCode":2000}" and the ContentType is "application/json" as it should be.

I'm OK with running the code that works, but I would be happy to know why deserialization by RestSharp is not happening correctly.