Notice error with push array on SplFixedArray

SplFixedArrayTest.php

<?php

$a = new SplFixedArray(1);
$a[0] = array();

$a[0][] = 'test'; // notice error

// ---------------

$a = new SplFixedArray(1);
$a[0] = array();

$tmp = $a[0];
$tmp[] = 'test';

$a[0] = $tmp;