Kan ikke trække ExtensionAttributes ud af AD

Koden ser sådan ud:

public class lookup2
{
  private String _givenname; public String Fornavn { get { return _givenname; } }
  private String _sn; public String Efternavn { get { return _sn; } }
  private String _extensionattribute15; public String EAN { get { return _extensionattribute15; } }
  private String _error; public String error { get { return _error; } }

  public DirectorySearcher DSESearcher;
  public String mySearchRoot;
  public DirectoryEntry entry;
  public DirectorySearcher myDirectorySearcher;

  public lookup2(String userid)
  {
      DSESearcher = new DirectorySearcher();
      mySearchRoot = DSESearcher.SearchRoot.Path;
      entry = new DirectoryEntry(mySearchRoot);
      myDirectorySearcher = new DirectorySearcher(entry);
      myDirectorySearcher.Filter = "(samaccountname=" + userid + ")";

      // Get the first entry of the search.
      SearchResult mySearchResult = myDirectorySearcher.FindOne();

      // Get the 'DirectoryEntry' that corresponds to 'mySearchResult'.
      DirectoryEntry myDirectoryEntry = mySearchResult.GetDirectoryEntry();

      // Get the properties of the 'mySearchResult'.
      try
      {
          ResultPropertyCollection arrAD;
          arrAD = mySearchResult.Properties;
          this._givenname = arrAD["givenname"][0].ToString();
          this._sn = arrAD["sn"][0].ToString();
          //Denne linje fejler:
          this._extensionattribute15 = arrAD["extensionattribute15"][0].ToString();
          this._error = "";
      }
      catch (Exception e)
      {
          this._error = e.ToString();
      }
  }
}

Det virker nu