ArrayAccess and "undefined" key

gistfile1.php

<?php

class foo implements arrayAccess
{
	public $values;

	function offsetSet($key, $value)
	{
		var_dump($key);

		$this->values[$key ?: sizeof($this->values)] = $value;
	}

	function offsetGet($key)
	{
	}

	function offsetExists($key)
	{
	}

	function offsetUnset($key)
	{
	}
}

$foo = new foo;
$foo[] = 'bar';
var_dump($foo->values);