title: MemoryError at ... No exception message supplied
body: <p>I have a very simple ManyToMany relationship:</p>
  
  <pre><code>@python_2_unicode_compatible
  class Mot(BaseModel):
      groupes_categories = models.ManyToManyField(GroupeCategories, blank=True,
                                                  through='MotGroupeCategories',
                                                  symmetrical=False,
                                                  related_name='mot')
      texte = models.CharField(max_length=150)
  
      def __str__(self):
          return _(u'{}').format(self.texte)
  
  
  @python_2_unicode_compatible
  class GroupeCategories(models.Model):
      description = models.CharField(max_length=150)
      exemple = models.CharField(max_length=150, blank=True, default=None,
                                 null=True)
  
      def __str__(self):
          return _(u'{0}{1}').format(
              self.description,
              u' ({0})'.format(self.exemple) if self.exemple else u''
          )
  
  
  @python_2_unicode_compatible
  class MotGroupeCategories(models.Model):
      mot = models.ForeignKey(Mot, blank=False)
      groupe_categories = models.ForeignKey(GroupeCategories, blank=False)
  
      def __str__(self):
          return u'{} - {}'.format(self.mot.texte,
                                   self.groupe_categories.description)
  </code></pre>
  
  <p>I've done this many times, and everything works fine except when I want to edit via administration a <code>MotGroupeCategories</code>:</p>
  
  <pre><code>class MotGroupeCategoriesAdmin(admin.ModelAdmin):
  
      def mot_texte(self, obj):
          return obj.mot.texte
      mot_texte.allow_tags = True
      mot_texte.short_description = _(u'I.')
  
      fields = ('mot', 'groupe_categories')
      list_display = ('groupe_categories', 'mot_texte', 'id')
      list_display_links = list_display
      search_fields = ['description']
  
  
  admin.site.register(MotGroupeCategories, MotGroupeCategoriesAdmin)
  </code></pre>
  
  <p>Everything works fine except when I want to edit a <code>MotGroupeCategories</code>: all those admins urls like:</p>
  
  <pre><code>/admin/main/motgroupecategories/[id]/
  </code></pre>
  
  <p>result in </p>
  
  <pre><code>MemoryError at /admin/main/motgroupecategories/12060915/
  No exception message supplied
  Request Method: GET
  Request URL:        /admin/main/motgroupecategories/[id]/
  Django Version: 1.8.7
  Exception Type: MemoryError
  Exception Location: C:\Python27\lib\site-packages\django\forms\widgets.py
      in render_options, line 547
  </code></pre>
  
  <p>It's the first time I have such a strange problem and nothing like that on stackoverflow.</p>