title: TypeError while rendering: 'long' object is not iterable
body: <p>I have a few models with some ForeignKey relationship.</p>
  
  <p>Through forms.py I added a <code>ModelMultipleChoiceField</code> widget</p>
  
  <pre><code>class VariableFieldForm(ModelForm):
      custom_field = forms.ModelMultipleChoiceField(queryset=VariableField.objects.all(), widget=FilteredSelectMultiple("Custom Fields", is_stacked=False))
  
      class Meta:
          model = VariableField
  </code></pre>
  
  <p>and when trying to access a saved record <code>localhost:8000/admin/product/subcategory/3/</code> or saving a new record I get the following traceback</p>
  
  <pre><code>File "/opt/python2.6/lib/python2.6/site-packages/django/utils/encoding.py" in force_unicode
    71. s = unicode(s)
  File "/opt/python2.6/lib/python2.6/site-packages/django/forms/forms.py" in __unicode__
    408. return self.as_widget()
  File "/opt/python2.6/lib/python2.6/site-packages/django/forms/forms.py" in as_widget
    439. return widget.render(name, self.value(), attrs=attrs)
  File "/opt/python2.6/lib/python2.6/site-packages/django/contrib/admin/widgets.py" in render
    39. output = [super(FilteredSelectMultiple, self).render(name, value, attrs, choices)]
  File "/opt/python2.6/lib/python2.6/site-packages/django/forms/widgets.py" in render
    581. options = self.render_options(choices, value)
  File "/opt/python2.6/lib/python2.6/site-packages/django/forms/widgets.py" in render_options
    531. selected_choices = set([force_unicode(v) for v in selected_choices])
  
  Exception Type: TemplateSyntaxError at /admin/product/subcategory/3/
  Exception Value: Caught TypeError while rendering: 'long' object is not iterable
  </code></pre>
  
  <p>I am using Django 1.3 and MySQL</p>
  
  <p>Ideas on how to solve it?</p>