Error

Traceback (most recent call last):
  File "/<appengine_dir>/google_appengine/google/appengine/ext/webapp/_webapp25.py", line 703, in __call__
    handler.post(*groups)
  File "/<project_dir>/app/handlers/students.py", line 30, in post
    if (form.is_valid() and upload):
  File "/<appengine_dir>/google_appengine/lib/django_1_2/django/forms/forms.py", line 121, in is_valid
    return self.is_bound and not bool(self.errors)
  File "/<appengine_dir>/google_appengine/lib/django_1_2/django/forms/forms.py", line 112, in _get_errors
    self.full_clean()
  File "/<appengine_dir>/google_appengine/lib/django_1_2/django/forms/forms.py", line 267, in full_clean
    self._clean_fields()
  File "/<appengine_dir>/google_appengine/lib/django_1_2/django/forms/forms.py", line 282, in _clean_fields
    value = field.clean(value, initial)
  File "/<appengine_dir>/google_appengine/google/appengine/ext/db/djangoforms.py", line 780, in clean_for_property_field
    property_clean(prop, value)
  File "/<appengine_dir>/google_appengine/google/appengine/ext/db/djangoforms.py", line 670, in property_clean
    prop.validate(prop.make_value_from_form(value))
AttributeError: 'NoneType' object has no attribute 'validate'