Can't fetch entities from IdentityMap when using a foreign keys as identifiers

CREATE TABLE `foo` (
  `fooID` int(11) NOT NULL AUTO_INCREMENT,
  `fooReference` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`fooID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8 COMMENT='module=bug';

CREATE TABLE `fooLocale` (
  `fooID` int(11) NOT NULL,
  `locale` varchar(5) NOT NULL DEFAULT '',
  `title` varchar(150) DEFAULT NULL,
  PRIMARY KEY (`fooID`,`locale`),
  KEY `fk_foo2` (`fooID`),
  CONSTRAINT `fk_table1_foo2` FOREIGN KEY (`fooID`) REFERENCES `foo` (`fooID`) ON DELETE CASCADE ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='module=bug';

INSERT INTO `foo` (`fooID`, `fooReference`)
VALUES
	(1, 'foo1');

INSERT INTO `fooLocale` (`fooID`, `locale`, `title`)
VALUES
	(1, 'en_GB', 'Foo title test');

use Doctrine\ORM\Mapping as ORM;

/**
 * Model for foo
 *
 * @category Application
 * @package Bug
 * @subpackage Model
 * @ORM\Table(name="foo")
 * @ORM\Entity
 */
class Bug_Model_Foo
{
	/**
	 * @var integer fooID
	 * @ORM\Column(name="fooID", type="integer", nullable=false)
	 * @ORM\GeneratedValue(strategy="IDENTITY")
	 * @ORM\Id
	 */
	protected $_fooID = null;

	/**
	 * @var string fooReference
	 * @ORM\Column(name="fooReference", type="string", nullable=true, length=45)
	 */
	protected $_fooReference = null;

	/**
	 * @ORM\OneToMany(targetEntity="Bug_Model_FooLocale", mappedBy="_foo",
	 * cascade={"persist"})
	 */
	protected $_fooLocaleRefFoo = null;

	/**
	 * Constructor
	 *
	 * @param array|Zend_Config|null $options
	 * @return Bug_Model_Foo
	 */
	public function __construct($options = null)
	{
		$this->_fooLocaleRefFoo = new \Doctrine\Common\Collections\ArrayCollection();
	}

}

use Doctrine\ORM\Mapping as ORM;

/**
 * Model for fooLocale
 *
 * @category Application
 * @package Bug
 * @subpackage Model
 * @ORM\Table(name="fooLocale")
 * @ORM\Entity
 */
class Bug_Model_FooLocale
{
	/**
	 * @ORM\ManyToOne(targetEntity="Bug_Model_Foo")
	 * @ORM\JoinColumn(name="fooID", referencedColumnName="fooID")
	 * @ORM\Id
	 */
	protected $_foo = null;

	/**
	 * @var string locale
	 * @ORM\Column(name="locale", type="string", nullable=false, length=5)
	 * @ORM\Id
	 */
	protected $_locale = null;

	/**
	 * @var string title
	 * @ORM\Column(name="title", type="string", nullable=true, length=150)
	 */
	protected $_title = null;

}

$query = $entityManager->createQuery('
			SELECT 
				f, fl 
			FROM 
				Bug_Model_Foo f 
			JOIN 
				f._fooLocaleRefFoo fl');
		return $query->getResult();

( ! ) Notice: Undefined index: _foo in /var/www/vhost/core/htdocs/externals/Doctrine/lib/Doctrine/ORM/Internal/Hydration/ObjectHydrator.php on line 217
Call Stack
#	Time	Memory	Function	Location
1	0.0002	337752	{main}( )	../index.php:0
2	0.0848	7268724	Zend_Application->run( )	../index.php:26
3	0.0848	7268724	Zend_Application_Bootstrap_Bootstrap->run( )	../Application.php:366
4	0.0850	7268796	Zend_Controller_Front->dispatch( )	../Bootstrap.php:97
5	0.0908	7900416	Zend_Controller_Dispatcher_Standard->dispatch( )	../Front.php:954
6	0.0967	8209564	Zend_Controller_Action->dispatch( )	../Standard.php:295
7	0.0967	8213912	Bug_IndexController->indexAction( )	../Action.php:513
8	0.1936	11391900	Bug_Service_Bug->getFooLocaleList( )	../IndexController.php:35
9	0.1936	11394088	Doctrine\ORM\AbstractQuery->getResult( )	../Bug.php:41
10	0.1936	11394296	Doctrine\ORM\AbstractQuery->execute( )	../AbstractQuery.php:392
11	0.1945	11397960	Doctrine\ORM\Internal\Hydration\AbstractHydrator->hydrateAll( )	../AbstractQuery.php:594
12	0.1946	11398656	Doctrine\ORM\Internal\Hydration\ObjectHydrator->_hydrateAll( )	../AbstractHydrator.php:99
13	0.1946	11399876	Doctrine\ORM\Internal\Hydration\ObjectHydrator->_hydrateRow( )	../ObjectHydrator.php:140
14	0.1949	11405584	Doctrine\ORM\Internal\Hydration\ObjectHydrator->_getEntityFromIdentityMap( )	../ObjectHydrator.php:326