Error when querying Active Directory

        string principal = this.Context.User.Identity.Name;
        string filter = string.Format("(&(ObjectClass={0})(sAMAccountName={1}))", "person", principal);
        string domain = "DC=ca,DC=com"; string[] properties = new string[] { "fullname" };

        DirectoryEntry adRoot = new DirectoryEntry("_LDAP://" + domain, null, null, AuthenticationTypes.Secure);
        DirectorySearcher searcher = new DirectorySearcher(adRoot);
        searcher.SearchScope = SearchScope.Subtree;
        searcher.ReferralChasing = ReferralChasingOption.All;
        searcher.PropertiesToLoad.AddRange(properties);
        searcher.Filter = filter;
        SearchResult result = searcher.FindOne();
        DirectoryEntry directoryEntry = result.GetDirectoryEntry();

        string displayName = directoryEntry.Properties["displayName"][0].ToString();
        string firstName = directoryEntry.Properties["givenName"][0].ToString();
        string lastName = directoryEntry.Properties["sn"][0].ToString();
        string email = directoryEntry.Properties["mail"][0].ToString();