Doctrine query one entity in one-to-many unidirectional with join table

I have two entities linked by a one-to-many unidirectional with join table association.

use Doctrine\ORM\Mapping as ORM;

/**
 * @Entity(repositoryClass="FooRepository")
 */
class Foo
{
    /**
     * @var Collection
     * @ORM\ManyToMany(targetEntity="Bar")
     * @ORM\JoinTable(
     *      name="foo_bar",
     *      inverseJoinColumns={@ORM\JoinColumn(unique=true)}
     * )
     */
    private $bars;

    /**
     * @var int
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    // [...]
}

/**
 * @Entity(repositoryClass="BarRepository")
 */
class Bar
{
    /**
     * @var int
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    // [...]
}

I'd like to create a method into my BarRepository class using the a foo id and a bar id which return one or null Bar object.

Actually my class looks like this:

use Doctrine\ORM\EntityRepository;

class BarRepository extends EntityRepository
{
    /**
     * Finds a single bar.
     * @param int $fooId The foo identifier.
     * @param int $barId The bar identifier.
     * @return Bar|null
     */
    public function findOneByFoo($fooId, $barId)
    {
        $qb = $this->createQueryBuilder('b');
        $qb->innerJoin('Foo', 'f', Expr\Join::WITH, 'f.id = :fooId')
        ->where('b.id = :barId')
        ->setParameter('fooId', $fooId)
        ->setParameter('barId', $barId)
        ->getQuery()->getOneOrNullResult();
    }
}

But this always returns a bar object even if the bar id is not associated to the foo object.