Django formset validation strange behavior

Question

I got some strange behavior with formset validation for model form formset. I'am using Django 1.5, python 2.7 Here is code what i used.

Model class:

class WorkDone(models.Model):
    task = models.ForeignKey(Task)
    type_of_work = models.ForeignKey("pricelist.TypeOfWork")

Form class:

class WorkDoneForm(forms.ModelForm):

    class Meta:
        model = WorkDone
        exclude = ("task",)

    def __init__(self, *args, **kwargs):
        super(WorkDoneForm, self).__init__(*args, **kwargs)
        self.fields["type_of_work"].widget = forms.HiddenInput()

Formset class generation:

WorkDoneFormSet = formset_factory(WorkDoneForm, can_delete=True)

Formset creation in get_context_data function of view:

context["work_done_formset"] = context.get("work_done_formset", WorkDoneFormSet(prefix='work_done'))

So, i submit form with one WorkDone form in formset without any entered data(for typeOfWork of course) and here POST data from POST request:

QueryDict: {u'_save': [u''],  
    u'work_done-0-type_of_work': [u''],  
    u'work_done-INITIAL_FORMS': [u'0'], 
    u'work_done-TOTAL_FORMS': [u'1'], 
    u'csrfmiddlewaretoken': [u'nFkTCyx3413yrFE9XpNQDGdNlzPAHwyI'], 
    u'work_done-MAX_NUM_FORMS': [u'1000']}

And output from this code bellow in view:

work_done_formset = WorkDoneFormSet(request.POST, prefix="work_done")
print work_done_formset.is_valid(), work_done_formset.cleaned_data
for form in work_done_formset:
    print form.is_valid(), form.cleaned_data

>> True [{}]
>> True {}

Why validation is true and why there is no error for missing TypeOfWork fields in form? Thank you.