APC __PHP_Incomplete_Class_Name

I am studying the use of APC in PHP. I created this script to check it out:

<?php

require 'Person.php';

if (!$p = apc_fetch('p')) {
    $p = new Person('larry');
    apc_store('p', $p);
}

echo $p->getName();

where Person.php:

<?php

class Person
{
    private $_name;

    public function __construct($_name)
    {
        $this->_name = $_name;
    }

    /**
     * @param mixed $name
     */
    public function setName($_name)
    {
        $this->_name = $_name;
    }

    /**
     * @return mixed
     */
    public function getName()
    {
        return $this->_name;
    }
}

It works fine, but the stored value in apc:

__PHP_Incomplete_Class::__set_state(array(
'__PHP_Incomplete_Class_Name' => 'Person',
'_name' => 'larry',
))

this is normal? or something is wrong with '__PHP_Incomplete_Class_Name'

thank