unique_together is not working in ClusterableModel about django-modelcluster HOT 3 CLOSED

@register_snippet
class NavigationMenu(ClusterableModel):
    FOOTERCOL1 = 'FC1'
    FOOTERCOL2 = 'FC2'
    FOOTERCOL3 = 'FC3'
    FOOTERCOL4 = 'FC4'
    MENU_CHOICES = (
        (FOOTERCOL1, 'Footer culumn 1'),
        (FOOTERCOL2, 'Footer culumn 2'),
        (FOOTERCOL3, 'Footer culumn 3'),
        (FOOTERCOL4, 'Footer culumn 4'),
    )

    objects         = NavigationMenuManager()
    menu_location   = models.CharField(null=False, max_length=255, help_text="Template name (do not change)", choices=MENU_CHOICES)
    site            = models.ForeignKey(Site, blank=True, null=True)
    menu_name       = models.CharField(null=True, blank=True, max_length=255)

    unique_together = ("site", "menu_location")

@register_snippet
class NavigationMenu(models.Model):
    FOOTERCOL1 = 'FC1'
    # ...
    class Meta:
        unique_together = [
            ("site", "menu_location"),
        ]