Set item in inventory to selected slot

I want to set item to 3rd slot in chest with InventoryContentPacket, but it always set to first slot :/

PHP:

$pk2 = new \pocketmine\network\mcpe\protocol\InventoryContentPacket;
$pk2->windowId = $ac->windowId;
$pk2->items[3] = $ac->oldItem;
$player->dataPacket($pk2);

 

Yeah, it's become pretty annoying since 1.2

PHP:

$pk2->items = array_fill(0, 2, Item::get(Item::AIR, 0, 0));
$pk2->items[3] = $ac->oldItem;