Problem with $eligible

When I use $eligible `$topic->broadcast(json_encode(['content' => $event->message]), [], [$client->getId()]);` then the WebSocket server is going shut-down because
> ErrorException: Object of class SplObjectStorage could not be converted to int in .../vendor/ipub/websockets-wamp/src/IPub/WebSocketsWAMP/Entities/Topics/Topic.php:95
