IndexOutOfRangeException: Index was outside the bounds of the array

IndexOutOfRangeException: Index was outside the bounds of the array

string propertyName = "mail";
               string User = HttpContext.Current.User.Identity.Name; // System.Environment.UserName;

               string[] Name = Regex.Split(User.Trim(), @"\\");
               string username = Name[1];
               //string domainname = HttpReq//System.Environment.UserDomainName.ToString().ToLower();
               string domainname = Name[0]; //"AsiaPacific";
               DirectoryEntry entry = new DirectoryEntry("LDAP://DC=" + domainname + ",DC=cpqcorp,DC=net");
               DirectorySearcher search = new DirectorySearcher(entry);
               search.Filter = "(&(&(objectClass=user)(SamAccountName=" + username + ")))";
               search.PropertiesToLoad.Add(propertyName);
               SearchResult result = search.FindOne();
               string propertyValue = "";
               if (result != null)
               {
                   propertyValue = result.Properties[propertyName][0].ToString();
                   //propertyValue = result.Properties.Count.ToString();

               }
               return propertyValue;

Exception Error:
[IndexOutOfRangeException: Index was outside the bounds of the array.]
   clsCommon.DisplayName()