Error in deserialization

[OperationContract]
        private System.Collections.Generic.List<DirectoryInfo> GetSubDirectories(string directory)
        {
            try
            {
                DirectoryInfo directoryPath = new DirectoryInfo(directory);
               // DirectoryInfo dir = new DirectoryInfo(directoryPath);
                List<DirectoryInfo> result = new List<DirectoryInfo>();
                //List<DirectoryInfo> resultChild = new List<DirectoryInfo>();
                DirectoryInfo[] childDirectories;

                childDirectories = directoryPath.GetDirectories();

                //HelpInfo help = new HelpInfo();
                //help.Count = 0;
                foreach (DirectoryInfo childDirectory in childDirectories)
                {
                    // Add the child directory to the result list

                    result.Add(childDirectory);

                }
                // return the full list of all subdirectories of the one passed in.
                return result;
            }
            catch (Exception ex) { DPS.Logging.LogMessage(NLog.LogLevel.Error,"Error Message -"+ex.Message+" "+"Error InnerException"+ ex.InnerException);}
            return null;
        }