Getting OAuth2 Update Token

I am trying to use the Google Calendar API to demonstrate the OAuth2 integration that we will need to do with another third party. I use the DotNetOpenAuth library, and I managed to get the initial redirect to Google to request Allow / Deny and return the authorization code.

Now I need to get the access token and update the token, but it seems to me that I get the access token back, refresh token is null.

This is my controller action method when Google redirects back after the user accepts or rejects:

public ActionResult ProcessResponse(string state, string code, string error)
{
  var oAuthClient =
    new WebServerClient(
      new AuthorizationServerDescription
      {
        TokenEndpoint = new Uri("https://accounts.google.com/o/oauth2/token"),
        AuthorizationEndpoint = new Uri("https://accounts.google.com/o/oauth2/auth"),
        ProtocolVersion = ProtocolVersion.V20
      },
      _applicationId, 
      _secret)
      {
        AuthorizationTracker = new TokenManager()
      };


  var authState = oAuthClient.ProcessUserAuthorization();

  var accessToken = authState.AccessToken;
  var refreshToken = authState.RefreshToken;

  return View(new[] { accessToken, refreshToken });
}

Any ideas?

EDIT:

To get the authorization code, I set oAuthClient identical to what I did above and use this method:

oAuthClient.RequestUserAuthorization(new[] { "https://www.googleapis.com/auth/calendar" }, returnUrl);