title: Django model choice field Select a valid choice. That choice is not one of the available choices
body: <p>I have a model choice field(Many to many relation), When I try to submit the form it throws an error <code>Select a valid choice. That choice is not one of the available choices.</code></p>
  
  <p>I have also added in <code>forms __init__</code> but still facing same error. I think the error is due to string, integer in checkbox value. Can anyone help me to fix this one. Posting my code here</p>
  
  <pre><code>class CustomSignupForm(forms.ModelForm):
      telephone = forms.CharField(label=_('Telephone'), required=False)
      def __init__(self, *args, **kwargs):
          self.request = kwargs.pop('request', None)
          super(CustomSignupForm, self).__init__(*args, **kwargs)
          self.fields['categories'] = forms.ModelChoiceField(queryset=Category.objects.filter(status=True), widget=forms.CheckboxSelectMultiple)
          for name, field in self.fields.iteritems():
              attr = {'class': 'custom-class'}
              if field.label:
                  attr['placeholder'] = field.label
              field.widget.attrs.update(attr)
  </code></pre>
  
  <p>Categories is the field with many to many selection, which is not allowing me to submit the form.</p>
  
  <p><strong>Model</strong></p>
  
  <pre><code>class Category(DateUpdate):
      title = models.CharField(max_length=256,null=True,blank=False)
      status = models.BooleanField(default=True)
      def __unicode__(self):
          return self.title
  </code></pre>
  
  <p><strong>Form</strong></p>
  
  <pre><code>&lt;div class="form-group"&gt;
      &lt;label&gt;
          {{ form.categories.label }}
      &lt;/label&gt;
      {% for check_choice in form.categories %}
              {{ check_choice.tag }}
              {{check_choice.choice_label}}
      {% endfor %}
      {% if form.categories.errors %}
          &lt;span class="error"&gt;
              {{ form.categories.errors.as_text }}
          &lt;/span&gt;
      {% endif %}
  &lt;/div&gt;
  </code></pre>