title: Django admin: create new user fails with extended user
body: <p>I'm using Django 1.6, but this error existed in 1.5.</p>
  
  <p>I have my own user class, which has no username field, only email.</p>
  
  <p>In my settings.py I have AUTH_USER_MODEL updated, everything works fine, when I want to authenticate or i.e. edit existing user in Django Admin site.</p>
  
  <p>But when I want to <strong>add</strong> new, not existing user, I have the following error:</p>
  
  <pre><code>TypeError at /admin/users/account/add/
  'int' object is not iterable
  Request Method: GET
  Request URL:    http://localhost:8000/admin/users/account/add/
  Django Version: 1.6
  Exception Type: TypeError
  Exception Value:    
  'int' object is not iterable
  Exception Location: /home/kaczor/git/panzer4/env/local/lib/python2.7/site-packages/django/forms/widgets.py in render_options, line 526
  </code></pre>
  
  <p>...</p>
  
  <pre><code>In template /home/kaczor/git/panzer4/env/local/lib/python2.7/site-packages/django/contrib/admin/templates/admin/includes/fieldset.html, error at line 19
  'int' object is not iterable
  9               {% for field in line %}
  10                  &lt;div{% if not line.fields|length_is:'1' %} class="field-box{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% if not field.is_readonly and field.errors %} errors{% endif %}"{% elif field.is_checkbox %} class="checkbox-row"{% endif %}&gt;
  11                      {% if not line.fields|length_is:'1' and not field.is_readonly %}{{ field.errors }}{% endif %}
  12                      {% if field.is_checkbox %}
  13                          {{ field.field }}{{ field.label_tag }}
  14                      {% else %}
  15                          {{ field.label_tag }}
  16                          {% if field.is_readonly %}
  17                              &lt;p&gt;{{ field.contents|linebreaksbr }}&lt;/p&gt;
  18                          {% else %}
  **19                                {{ field.field }}**
  20                          {% endif %}
  21                      {% endif %}
  22                      {% if field.field.help_text %}
  23                          &lt;p class="help"&gt;{{ field.field.help_text|safe }}&lt;/p&gt;
  24                      {% endif %}
  25                  &lt;/div&gt;
  26              {% endfor %}
  27          &lt;/div&gt;
  28      {% endfor %}
  29  &lt;/fieldset&gt;
  </code></pre>
  
  <p>my model looks like:</p>
  
  <pre><code>class Account(AbstractBaseUser, PermissionsMixin):
  
      email = models.CharField(max_length=40, unique=True, blank=False, db_index=True)
      first_name = models.CharField(max_length=64, null=True)
      last_name = models.CharField(max_length=64, null=True)
      is_staff = models.BooleanField(default=False)
      is_active = models.BooleanField(default=True)
      categories_allowed = TreeManyToManyField(Category, default=1)
  
      objects = AccountManager()
  
      def save(self, *args, **kwargs):
          if not self.email or self.email == '':
              raise ValueError(NO_EMAIL_ERROR)
          else:
              super(Account, self).save(*args, **kwargs)
  
      def __unicode__(self):
          return "%s %s" % (self.first_name, self.last_name)
  
      USERNAME_FIELD = 'email'
      REQUIRED_FIELDS = []
  </code></pre>
  
  <p>Django Admin allows to edit the user, but fail whenI click on <em>add</em> button.
  What is wrong with that? </p>