[New API] Problem getting total players in world

I have a stange problem getting the number of online players in world. This is my code:

PHP:

$player->sendMessage("Players in world" . count($sender->getLevel()->getPlayers()));


The code works but i get this thing in PocketMine console: "[NOTICE] An E_NOTICE error happened: "Undefined offset: 1 in...".
What did I do wrong?

Undefined offset in what? That's critical...