TypeError: argument 3 must be string or read-only buffer, not None

[falkb]

On web interface page Admin-->Permission, try to add a new subject to a group. The result will be:

Datei "build/bdist.win32/egg/trac/web/main.py", Zeile 511, in _dispatch_request
  dispatcher.dispatch(req)
Datei "build/bdist.win32/egg/trac/web/main.py", Zeile 237, in dispatch
  resp = chosen_handler.process_request(req)
Datei "build/bdist.win32/egg/trac/admin/web_ui.py", Zeile 116, in process_request
  path_info)
Datei "build/bdist.win32/egg/trac/admin/web_ui.py", Zeile 357, in render_admin_panel
  for action in perm.get_user_permissions(group):
Datei "build/bdist.win32/egg/trac/perm.py", Zeile 372, in get_user_permissions
  for perm in self.store.get_user_permissions(username) or []:
Datei "build/bdist.win32/egg/trac/perm.py", Zeile 173, in get_user_permissions
  subjects.update(provider.get_permission_groups(username) or [])
Datei "build/bdist.win32/egg/ldapplugin/api.py", Zeile 128, in get_permission_groups
  ldapgroups = self._get_user_groups(username)
Datei "build/bdist.win32/egg/ldapplugin/api.py", Zeile 177, in _get_user_groups
  if self._ldap.is_in_group(userdn, group):
Datei "build/bdist.win32/egg/ldapplugin/api.py", Zeile 548, in is_in_group
  cr = self._compare(groupdn, self.groupmember, userdn)
Datei "build/bdist.win32/egg/ldapplugin/api.py", Zeile 656, in _compare
  cr = self._ds.compare_s(dn, attribute, value)
Datei "D:/bitnami-trac-0.12.2/python/Lib/site-packages/ldap/ldapobject.py", Zeile 266, in compare_s
  return self.compare_ext_s(dn,attr,value,None,None)
Datei "D:/bitnami-trac-0.12.2/python/Lib/site-packages/ldap/ldapobject.py", Zeile 253, in compare_ext_s
  msgid = self.compare_ext(dn,attr,value,serverctrls,clientctrls)
Datei "D:/bitnami-trac-0.12.2/python/Lib/site-packages/ldap/ldapobject.py", Zeile 250, in compare_ext
  return self._ldap_call(self._l.compare_ext,dn,attr,value,RequestControlTuples(serverctrls),RequestControlTuples(clientctrls))
Datei "D:/bitnami-trac-0.12.2/python/Lib/site-packages/ldap/ldapobject.py", Zeile 100, in _ldap_call
  result = func(*args,**kwargs)