title: 'NOT NULL constraint failed' after adding to models.py
body: <p>I'm using userena and after adding the following line to my models.py</p>
  
  <pre><code>zipcode = models.IntegerField(_('zipcode'),
                                     max_length=5)
  </code></pre>
  
  <p>I get the following error after I hit the submit button on th signup form:</p>
  
  <pre><code>IntegrityError at /accounts/signup/
  NOT NULL constraint failed: accounts_myprofile.zipcode
  </code></pre>
  
  <p>My question is what does this error mean, and is this related to Userena?</p>