title: Django DateTimeField TypeError: expected string or bytes-like object
body: <p>I know there are many posts about this error. Such as: <a href="https://stackoverflow.com/questions/36737221/expected-string-or-buffer-date-re-matchvalue-django-error">this</a> and <a href="https://stackoverflow.com/questions/27220480/django-datetime-migration-error">this one</a></p>
  
  <p>But I knew already that. And that's why I'm getting crazy. </p>
  
  <p>When I create an instance and try to save it, I don't have problems:</p>
  
  <pre><code>per_detail.preview_title = per_detail.details_sample.preview_title
  per_detail.icon = per_detail.details_sample.icon
  per_detail.content = fill_content(per_detail)
  per_detail.save()
  </code></pre>
  
  <p>easy.</p>
  
  <p>But when there's already the instance, and I try to save it again (updated it). Then I get this error:</p>
  
  <blockquote>
  <pre><code>match = datetime_re.match(value)
  </code></pre>
    
    <p>TypeError: expected string or bytes-like object</p>
  </blockquote>
  
  <p>whit this code:</p>
  
  <pre><code>personal_detail_sample = kwargs['instance']
          personal_details = PersonalDetail.objects.filter(Q(details_sample=personal_detail_sample))
          for per_detail in personal_details:
              per_detail_updated = fill_updated_content(personal_detail_sample, per_detail)
              per_detail_updated.save()
  </code></pre>
  
  <p>I gotta say, it doesn't matter where I try to update that instance, I get always the same error. (so, it's not because the <code>kwargs['instance']</code>)</p>
  
  <p>And here's the field, that is giving troubles:</p>
  
  <p><code>sent_date = models.DateTimeField(_('sent_date'), null=True, blank=True)</code></p>
  
  <p>As you maybe noticed, I never filled the field "sent_date", but it should be null. So it shouldn't be a problem. And for just being safe, I also tried to do: </p>
  
  <pre><code>per_detail.sent_date = *a date*
  per_detail.save()
  </code></pre>
  
  <p>And I'm getting the same error. </p>
  
  <p>I don't have any idea what could it be. </p>
  
  <p>Maybe someone can help me. </p>