PHPUnit's assertNull() failing

gistfile1.txt

<?php
class Unprintable
{
    public function __construct($parent = NULL, $nestingLimit = 4)
    {
        $this->parent = $parent;
        if ($nestingLimit == 0) {
            return;
        }
        for ($i = 0; $i < 10; $i++) {
            $field = 'field' . $i;
            $this->$field = new Unprintable($this, $nestingLimit - 1);
        }
    }
}

class UnprintableTest extends PHPUnit_Framework_TestCase
{
    public function setUp()
    {
        ini_set('memory_limit', '32M');
    }

    public function testObjectCanBeBuilt()
    {
        $object = new Unprintable();
        $this->assertInstanceOf('Unprintable', $object);
    }

    public function testObjectCanBeAsserted()
    {
        $object = new Unprintable();
        $this->assertNull($object);
    }
}