Django ModelForm individual fields not validated when overriding clean() on Model - django

Question

I am using standard Django Models and ModelForms.
On the Model, I am overriding the clean() method to check that 2 fields in combination are valid. I am having validators[], not nulls etc defined on other fields.
In the ModelForm, I am not modifying anything. I only set the corresponding Model in the ModelForm's Meta class.
When I try to create a new object through the form, the individual fields in the ModelForm/Model are not validated when I call form.is_valid() in the View. According to the docs (https://docs.djangoproject.com/en/1.6/ref/models/instances/#validating-objects) the is_valid() method on the Form should call the Model's clean_fields() method (first).
This doesn't seem to work when you submit a form without a Model instance (or a new instance not in the db). When I'm editing an existing object, all is well. It nicely triggers invalid values in individual fields before calling the Model's clean() method.
When I remove the overridden clean() method from my Model, all is well. Individual fields are validated both when creating new objects and editing existing ones.
I have also tested this with the admin module's forms. It has exactly the same behaviour.
So the question is, why does overriding the clean() method on my Model prevent the ModelForm validating the individual fields before calling the clean() method to test additional cross-field stuff???
Note that I am not validating the ModelForm. All validation is on the Model itself.
Model:
class Survey(models.Model):
from_date = models.DateField(null=False, blank=False, validators=[...])
to_date = models.DateField(null=False, blank=False, validators=[...])
(...)
def clean(self):
errors = []
# At this point I expect self.to_date already to be validated for not null etc.
# It isn't for new Model instances, only when editing an existing one
if self.to_date < self.from_date:
errors.append(ValidationError("..."))
ModelForm:
class TestForm(ModelForm):
class Meta:
model = Survey
View (to render blank form for new Model data entry):
(...)
if request.method == "POST":
survey_form = TestForm(request.POST)
if '_save' in request.POST:
if survey_form.is_valid():
survey_form.save()
return HttpResponseRedirect(next_url)
else:
return HttpResponseRedirect(next_url)
else:
survey_form = TestForm()
context = {'form': survey_form}
(...)