title: Argument of type 'NoneType' is not iterable with ModelForm
body: <p>I'm getting the following error when trying to submit a form:</p>
  
  <p><code>argument of type 'NoneType' is not iterable</code></p>
  
  <p>My form class is as follows:</p>
  
  <pre><code>class requestform(ModelForm):
  def __init__(self, *args, **kwargs):
      super(requestform, self).__init__(*args,**kwargs)
      self.is_update=False
  requestedpart = forms.CharField (label="Requested Part", max_length=80, required= True)
  librarytype = forms.TypedChoiceField (label="Type", choices = ((1, "Type1"), (2, "Type2"), (3, "Type3")), coerce = lambda x: bool(int(x)), widget = forms.RadioSelect, required= True)
  requestformat = forms.TypedChoiceField (label="Format", choices = ((1, "Format1"), (2, "Format2")), coerce = lambda x: bool(int(x)), widget = forms.RadioSelect, required= True)
  def clean(self):
      if 'requestedpart' not in self.cleaned_data:
          return
      if not self.is_update:
          return self.cleaned_data
  class Meta:
      model = LibraryRequest
  </code></pre>
  
  <p>My views is as follows</p>
  
  <pre><code>if request.method == "POST":
      formtoaddrequest = requestform(request.POST, request.FILES)
      if requestform.is_valid(formtoaddrequest):
          new_request = formtoaddrequest.save(commit=False)
          new_request.requestadder = request.user 
          new_request.save()
          request.user.message_set.create(message=_("You have saved '%(requestedpart)s'") %  {'requestedpart': new_request.requestedpart})
  </code></pre>
  
  <p>Here's my traceback</p>
  
  <pre><code>Traceback:
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    100.                     response = callback(request, *callback_args, **callback_kwargs)
  File "/Users/AB/Desktop/store/project/views.py" in view_requests
    201.          if formtoaddmodel.is_valid():
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/forms/forms.py" in is_valid
    121.         return self.is_bound and not bool(self.errors)
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/forms/forms.py" in _get_errors
    112.             self.full_clean()
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/forms/forms.py" in full_clean
    269.         self._post_clean()
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/forms/models.py" in _post_clean
    316.         self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
  File "/Users/AB/Desktop/myenv2/lib/python2.7/site-packages/django/forms/models.py" in construct_instance
    40.                 or not f.name in cleaned_data:
  
  Exception Type: TypeError at /project/view_requests/
  Exception Value: argument of type 'NoneType' is not iterable
  </code></pre>
  
  <p>Any insight into why I might be getting this error?</p>