Unable to add entity in @ManyToMany association when owning entity is extended from another entity

/**
 * @Entity
 * @HasLifecycleCallbacks
 * @InheritanceType("JOINED")
 * @DiscriminatorColumn(name="discr", type="string")
 * @DiscriminatorMap({"guest" = "Shop_Data_Entity_Guest", "customer" = "Shop_Data_Entity_Customer"})
 */
class Shop_Data_Entity_Guest extends Shop_Data_Entity_Abstract {
    public function __construct() {
        /* call parent construct with all parameters */
        call_user_func_array(array("parent","__construct"),func_get_args());
    }
}

/**
 * @Entity
 * @HasLifecycleCallbacks
 */
class Shop_Data_Entity_Customer extends Shop_Data_Entity_Guest {
    /**
     * @ManyToMany(targetEntity="Shop_Data_Entity_Contact", cascade={"persist","remove"} )
     * @JoinTable(name="Shop_Data_Entity_Customer__homes",
     *      joinColumns={@JoinColumn(name="customer_id", referencedColumnName="id", onDelete="cascade" )},
     *      inverseJoinColumns={@JoinColumn(name="contact_id", referencedColumnName="id", onDelete="cascade" )}
     *      )
     */
    public $homes;

    public function __construct() {
        /* call parent construct with all parameters */
        call_user_func_array(array("parent","__construct"),func_get_args());

        $this->homes = new Shop_Data_Collection_Standard(); // this class only extends ArrayCollection
    }
}

        $q = $em->createQuery("select c from Shop_Data_Entity_Customer c");
        $q->setMaxResults(1);
        $customers = $q->getResult();
        
        $contact = new Shop_Data_Entity_Contact();
        $customers[0]->home->add($contact);
        $em->flush();

/** @MappedSuperclass */
class Shop_Data_Entity_Guest extends Shop_Data_Entity_Abstract {
    public function __construct() {
        /* call parent construct with all parameters */
        call_user_func_array(array("parent","__construct"),func_get_args());
    }
}