Provider Hosted App note getting context token

Hi,

Here's a sample code that we are using for a provider hosted app. Somehow the contextToken is coming as null and hence not able to pull information from SP list. Please suggest.

try
               {
                   SharePointContextToken contextToken;
                   Uri sharepointUrl;
                   string accessToken;
                   TokenHelper.TrustAllCertificates();
                   string contextTokenString = TokenHelper.GetContextTokenFromRequest(Request);
                   if (contextTokenString != null)
                   {
                       contextToken = TokenHelper.ReadAndValidateContextToken(contextTokenString, Request.Url.Authority);
                       sharepointUrl = new Uri(Request.QueryString["SPHostUrl"]);
                       accessToken = TokenHelper.GetAccessToken(contextToken, sharepointUrl.Authority).AccessToken;
                       ClientContext context = TokenHelper.GetClientContextWithAccessToken(sharepointUrl.ToString(), accessToken);

SharePoint Architect