Django: empty form errors

I've been experiencing a little problem when I try to update some record from database. Strange thing is that form.errors are empty if form is invalid (I can't understand why).

Here are the

form

class PetitionUpdateForm(forms.ModelForm):
  owner = forms.ModelChoiceField(
    label=_('Petition creator'),
    queryset=User.objects.all(),
    widget=forms.HiddenInput()
  )

  class Meta:
    fields = ('title', 'petition_text', 'description',
              'category', 'num_signs', 'date_to', 'owner',)
    model = Petition

model

class Petition(models.Model):
  PETITION_STATUSES = (
    ('N', _('New petition')), # New one
    ('M', _('Moderation')),   # On moderation
    ('R', _('Rejected')),     # Failed petition
    ('S', _('Success'))       # Succeeded petition
  )

  title = models.CharField(max_length=512)
  slug = models.SlugField(max_length=512, editable=False, blank=True)
  description = models.TextField()
  petition_text = models.TextField(blank=True, null=True)
  petition_picture = models.ImageField(upload_to=get_upload_path, blank=True)
  petitioning = models.ManyToManyField(PetitionTarget, editable=False)
  signs = models.ManyToManyField(User, editable=False, related_name='petition_signs')
  num_signs = models.IntegerField(max_length=11, default=100, blank=True)
  category = models.ForeignKey(Category, blank=True, null=True, related_name='petition_category')
  date_to = models.DateTimeField(blank=True, null=True)

  videos = models.ManyToManyField(Video, editable=False)
  photos = models.ManyToManyField(Photo, editable=False)
  audios = models.ManyToManyField(Audio, editable=False)
  documents = models.ManyToManyField(Document, editable=False)

  created = models.DateTimeField(auto_now_add=True, editable=False)
  changed = models.DateTimeField(auto_now=True, editable=False)
  status = models.CharField(max_length=1, choices=PETITION_STATUSES, default='M', blank=True)
  owner = models.ForeignKey(User, related_name='petition_owner')

  def __unicode__(self):
    return u'{0}: {1}'.format(_('Petition'), self.title)

update view

@login_required
@render_to('petition/edit-petition.html')
def update_petition(request, slug):
  p = get_object_or_404(Petition, slug=slug)
  form = PetitionUpdateForm(request.POST or None, instance=p)
  import pdb
  pdb.set_trace()
  if form.is_valid():
    form.save()
    messages.success(request, _('Petition saved'))
  else:
    print form.errors # errors are empty
    messages.success(request, _('Some error happened'))

  return {'form': form, 'petition': p}

What's wrong with my code?

I've already tried to set null attributes for the most of model fields, switched from class based view to a standard view and yet I'm unable to update my record.

Sultan,

Thanks