title: Django __str__ returned non-string (type tuple), i didn't define __str__
body: <p>I'm working on a duplicate reporting system on Django, and i created a table on my models.py that goes</p>
  <pre><code>class DupReport(models.Model):
  count = models.IntegerField()
  oldersub = models.ForeignKey('Submission', on_delete=models.CASCADE, related_name='older_sub')
  newersub = models.ForeignKey('Submission', on_delete=models.CASCADE, related_name='newer_sub')
  </code></pre>
  <p>And when i go to admin to add a row, the menu that displays the rows works fine, but when i click add DupReport, i get this:</p>
  <blockquote>
  <p>TypeError at /admin/apppickoff/dupreport/add/</p>
  <p><strong>str</strong> returned non-string (type tuple)</p>
  <p>&lt; a href=&quot;{% add_preserved_filters history_url %}&quot; class=&quot;historylink&quot;&gt;{% trans &quot;History&quot; %}&lt; /a&gt;</p>
  </blockquote>