DirectoryEntry oDE = new DirectoryEntry("LDAP://DC=Test1,DC=Test2,DC=gov,DC=lk");

using (DirectorySearcher ds = new DirectorySearcher(oDE))
{
    ds.PropertiesToLoad.Add("name");
    ds.PropertiesToLoad.Add("userPrincipalName");

    ds.Filter = "(&(objectClass=user))";

    SearchResultCollection results = ds.FindAll();

    foreach (SearchResult result in results)
    {
        Console.WriteLine("{0} - {1}",
            result.Properties["name"][0].ToString(),
            result.Properties["userPrincipalName"][0].ToString());
    }
}

On the SearchResultCollection results = ds.FindAll(); line I get an exception:

A referral was returned from the server

Why do I get that exception and what does it mean?