A signal m2m_changed and bug with post_remove

I need to detect a post_remove signal, so I have written :

def handler1(sender, instance, action, reverse, model, pk_set, **kwargs):
if (action == 'post_remove'):
    test1()  # not declared but make a bug if it works, to detect :)

m2m_changed.connect(handler1, sender=Course.subscribed.through)

If I change 'post_remove' by 'post_add' it is ok.. Is it a django's bug about post_remove ??

I use that model and I switch beetween two values of 'subscribed' (so one added and one deleted)

class Course(models.Model):
    name = models.CharField(max_length=30)
    subscribed = models.ManyToManyField(User, related_name='course_list', blank=True, null=True, limit_choices_to={'userprofile__status': 'student'})

I have seen a post with a bug of django, maybe it havn't been fixed... (or it's me ^^)