hasManyToMany works in SQL but not in Phalcon

Phalcon version 1.2.4

class Server extends \Phalcon\Mvc\Model {
  public function initialize() {
    $this->hasManyToMany(
      'server_id', 'ServersSubscriptions', 'server_id', 'subscription_id', 'Subscription', 'subscription_id', 
      array('alias' => 'subscriptions')
    )
  };

  public function getSource() {
    return 'servers';
  }
}

class ServersSubscriptions extends \Phalcon\Mvc\Model {
  public function initialize() {
    $this->belongsTo('server_id', 'Server', 'server_id');
    $this->belongsTo('subscription_id', 'Subscription', 'subscription_id');
  }

  public function getSource() {
    return 'servers_subscriptions';
  }
}

class Subscription extends \Phalcon\Mvc\Model {
  public function initialize() {
    $this->hasManyToMany(
      'subscription_id', 'ServersSubscriptions', 'subscription_id', 'server_id', 'Server', 'server_id', 
      array('alias' => 'servers')
    )
  };
}

class Test {
  public function run() { 
    $server = Server::findFirst(1);
    $subscriptions = $server->subscriptions; 

    // result: $subscriptions is empty

    $subscriptions = $server->getRelated('subscriptions', null);

    // result: still empty
  }
}

but ...

SELECT * 
      FROM subscriptions 
INNER JOIN servers_subscriptions ON servers_subscriptions.subscription_id = subscriptions.subscription_id 
     WHERE servers_subscriptions.server_id = 1

returns one record, which is correct.

What am I doing wrong here?