PHP Iterator refcount crash

tree.php

<?php
$tree = array();
$branch = &$tree;

$courses = array(
    'f1' => '/d1',
    'f6' => '/d2',
    'f7' => '/d3',
    'f8' => '/d3/d4',
    'f9' => '/d3/d4',
);

foreach($courses as $id => $course) {
    $path   = explode('/', substr($course, 1));
    $branch = &$tree;
    foreach($path as $category) {
        if (!isset($branch[$category])) {
            $branch[$category] = array();
        }
        $branch = &$branch[$category];
    }
    $branch[] = $id;
}

$iterator = new RecursiveIteratorIterator(
    new RecursiveArrayIterator($tree),
    RecursiveIteratorIterator::SELF_FIRST
);

foreach($iterator as $file);
// exit; // uh ?

Crash without exit