How can I set IMP to display the user's mailbox quota in the webmail interface?

I tried adding this to the /etc/psa-horde/imp/servers.php file under the $servers['imap'] section but that results in just a "Unable to retrieve quota" message when I view the mailbox.

============================
'quota' => array(
'driver' => 'courier',
'params' => array(
'login' => '',
'password' => ''
)
),
============================


Thanks.