Php nModified返回空值

我正在使用新的PHP Mongo驱动程序执行批量更新操作。 文档会在集合中更新，但nModified值始终为null

use MongoDB\Driver\BulkWrite;
use MongoDB\Driver\Query as MongoDbQuery;
use MongoDB\Driver\Manager as MongoDbManager;

class Offers {

    const NS = 'ecommerce_v2.products';

    public function insert($offer, $merchant, $scope) {
        $manager = new MongoDbManager();
        $bulk = new BulkWrite();

        $filter = [
            'merchants_id' => (int) $merchant,
            'external_reference' => ['$in' => $scope]
        ];

        $bulk->update($filter, ['$set' => ['offer' => $offer]], ['multi' => true]);
        // MongoDB\Driver\WriteResult
        $result = $manager->executeBulkWrite(static::NS, $bulk);
        // NULL
        return $result->getModifiedCount();
    }
}

有什么想法吗