Hi guy, i dont know where is my error, but Django 1.2.1 is give this error: 'NoneType' object has no attribute 'day' when i try to save form from the Administrator Area

models.py

from django.db import models
from django.contrib.auth.models import User


class Editorial(models.Model):

    titulo = models.CharField(max_length=250,help_text='Titulo del editorial')
    editorial = models.TextField(help_text='Editorial')
    slug = models.SlugField(unique_for_date='pub_date')
    autor = models.ForeignKey(User)
    pub_date = models.DateTimeField(auto_now_add=True)
    activa = models.BooleanField(verbose_name="Activa")
    enable_comments = models.BooleanField(verbose_name="Aceptar Comentarios",default=False)

    editorial_html = models.TextField(editable=False,blank=True)

    def __unicode__(self):
        return unicode(self.titulo)

    def get_absolute_url(self):
        return "/editorial/%s/%s/" % (self.pub_date.strftime("%Y/%b/%d").lower(), self.slug)

    class Meta:
        ordering=['-pub_date']   
        verbose_name_plural ='Editoriales'

    def save(self,force_insert=False, force_update=False):
        from markdown import markdown
        if self.editorial:
            self.editorial_html = markdown(self.editorial)
        super(Editorial,self).save(force_insert,force_update)

i dont know why this error,

COMPLETED ERROR:

    Traceback:
File "C:\wamp\bin\Python26\lib\site-packages\django\core\handlers\base.py" in get_response
  100.                     response = callback(request, *callback_args, **callback_kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\contrib\admin\options.py" in wrapper
  239.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\utils\decorators.py" in _wrapped_view
  76.                     response = view_func(request, *args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\views\decorators\cache.py" in _wrapped_view_func
  69.         response = view_func(request, *args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\contrib\admin\sites.py" in inner
  190.             return view(request, *args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\utils\decorators.py" in _wrapper
  21.             return decorator(bound_func)(*args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\utils\decorators.py" in _wrapped_view
  76.                     response = view_func(request, *args, **kwargs)
File "C:\wamp\bin\Python26\lib\site-packages\django\utils\decorators.py" in bound_func
  17.                 return func(self, *args2, **kwargs2)
File "C:\wamp\bin\Python26\lib\site-packages\django\db\transaction.py" in _commit_on_success
  299.                     res = func(*args, **kw)
File "C:\wamp\bin\Python26\lib\site-packages\django\contrib\admin\options.py" in add_view
  777.             if form.is_valid():
File "C:\wamp\bin\Python26\lib\site-packages\django\forms\forms.py" in is_valid
  121.         return self.is_bound and not bool(self.errors)
File "C:\wamp\bin\Python26\lib\site-packages\django\forms\forms.py" in _get_errors
  112.             self.full_clean()
File "C:\wamp\bin\Python26\lib\site-packages\django\forms\forms.py" in full_clean
  269.         self._post_clean()
File "C:\wamp\bin\Python26\lib\site-packages\django\forms\models.py" in _post_clean
  345.             self.validate_unique()
File "C:\wamp\bin\Python26\lib\site-packages\django\forms\models.py" in validate_unique
  354.             self.instance.validate_unique(exclude=exclude)
File "C:\wamp\bin\Python26\lib\site-packages\django\db\models\base.py" in validate_unique
  695.         date_errors = self._perform_date_checks(date_checks)
File "C:\wamp\bin\Python26\lib\site-packages\django\db\models\base.py" in _perform_date_checks
  802.                 lookup_kwargs['%s__day' % unique_for] = date.day

Exception Type: AttributeError at /admin/editoriales/editorial/add/
Exception Value: 'NoneType' object has no attribute 'day'

thanks guys

sorry with my English