Django forms keyerror on ModelChoiceField

I have a form used for advanced searching. One of the fields, island_group is a ModelChoiceField for which I am returning a values_list as the queryset (so as to get each distinct value only once).

forms.py:

class SearchForm(forms.Form):
    ...
    island_group = forms.ModelChoiceField(
        required=False,
        queryset=Locality.objects.values_list('islandgroup', flat=True).distinct('islandgroup')
    ...

and because this throws an error along the lines of invalid choice because I'm using the values rather than the objects, I override the error:

    def clean(self):
        cleaned_data = super(SearchForm, self).clean()

        if self._errors["island_group"]:
            del self._errors["island_group"]

        return cleaned_data

However, I keep getting KeyError at /collections/ 'island_group' errors. In fact as this data is being pulled from a read-only database, I just want to be able to pass it into the views with minimum fuss.

Any help would be much appreciated. Also, here is the traceback:

Traceback:
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/sam/django-projects/datazone/bin/datazone/cdrs/views.py" in cdrs_index
  59.     return render(request, 'cdrs_index.html', context)    
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/shortcuts/__init__.py" in render
  44.     return HttpResponse(loader.render_to_string(*args, **kwargs),
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/loader.py" in render_to_string
  188.         return t.render(context_instance)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render
  123.             return self._render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/test/utils.py" in instrumented_test_render
  57.     return self.nodelist.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
  127.         return compiled_parent._render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/test/utils.py" in instrumented_test_render
  57.     return self.nodelist.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
  64.             result = block.nodelist.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render
  744.                 bits.append(self.render_node(node, context))
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render_node
  757.         return node.render(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
  227.                 nodelist.append(node.render(context))
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in render
  792.             output = self.filter_expression.resolve(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in resolve
  510.                 obj = self.var.resolve(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in resolve
  653.             value = self._resolve_lookup(context)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/template/base.py" in _resolve_lookup
  683.                         current = getattr(current, bit)
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/forms/forms.py" in _errors
  415.         return self.form.errors.get(self.name, self.form.error_class())
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/forms/forms.py" in _get_errors
  112.             self.full_clean()
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/forms/forms.py" in full_clean
  268.         self._clean_form()
File "/home/sam/django-projects/datazone/local/lib/python2.7/site-packages/django/forms/forms.py" in _clean_form
  296.             self.cleaned_data = self.clean()
File "/home/sam/django-projects/datazone/bin/datazone/cdrs/forms.py" in clean
  113.         if self._errors["island_group"]:

Exception Type: KeyError at /collections/
Exception Value: 'island_group'