Roundcube не работает в связке с memcache

# apt-cache show php5-memcache | grep -i version
Version: 3.0.6-6
# apt-cache show memcached | grep -i version
Version: 1.4.13-0.2+deb7u1

# vim /var/www/roundcube/config/config.inc.php
$rcmail_config['session_storage'] = 'memcache';
$rcmail_config['memcache_hosts'] = array('localhost:11211');

# tail -1 /var/www/roundcube/logs/errors
[06-Jul-2014 14:15:24 +0400]: DB Error: Failed to connect to memcached. Please check configuration in /var/www/roundcube/program/lib/Roundcube/rcube_session.php on line 83 (GET /webmail/)