ldap filter `memberOf` does not work

I use glauth w/ ldap backend. The filter using `memberOf` doesn't seem to work. Example:
```
ldapsearch -H ldaps://ldap-proxy.example.com:636 -b dc=example,dc=com -D uid=abc,cn=users,dc=example,dc=com -x -w PASS '(memberOf=cn=admin,cn=groups,dc=example,dc=com)'
```

The same query works with the ldap backend.