DirectoryEntry.Invoke("SetPassword", .... The directory property cannot be found in the cache

Hello,
  I'm having trouble changing a user's password in LDAP using ASP .Net 2.0 / C#.

The LDAP server is: OpenLDAP on Red Hat 4EL.


The code is as follows:

de = new DirectoryEntry(ldapPath, ldapUsername, ldapPassword);
de.AuthenticationType = AuthenticationTypes.ServerBind;

DirectorySearcher srch = new DirectorySearcher(de);
srch.Filter = "(CN=user.name)";

SearchResult result = srch.FindOne();

if (result != null)
{
     // User found, try to reset their password
     try
     {
          DirectoryEntry user = result.GetDirectoryEntry();
          user.Invoke("setPassword", new object[] { newPassword });
          de.CommitChanges();
     }
.
.
.

The error occurs at this line:
          user.Invoke("setPassword", new object[] { newPassword });

and returns these exceptions:
          Exception.Message = Exception has been thrown by the target of an invocation.
          Exception.InnerException.Message = The directory property cannot be found in the cache.

Thank you in advance for your help!

Tim