group by email address在php中不能与mongodb一起使用

$m = new MongoDB\Client("mongodb://localhost:27017");
$db = $m->mailchimp;   
$collection = $db->users;
$aggregate[] = [
    '$group' => [
        '_id' => 'email_address',  
        'email_address' => array('$sum'=>'1'),
    ],
];
$data=$collection->aggregate($aggregate);