title: inheritance from the django user model results in error when changing password
body: <p>I inherited form the django user model like so:</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import User, UserManager
  from django.utils.translation import ugettext_lazy as _
  
  class NewUserModel(User):
      custom_field_1 = models.CharField(_('custom field 1'), max_length=250, null=True, blank=True)
      custom_field_2 = models.CharField(_('custom field 2'), max_length=250, null=True, blank=True)
  
      objects = UserManager()
  </code></pre>
  
  <p>When i go to the admin and add an entry into this model, it saves fine, but below the "Password" field where it has this text "Use '[algo]$[salt]$[hexdigest]' or use the change password form.", if i click on the "change password form' link, it produces this error</p>
  
  <pre><code>Truncated incorrect DOUBLE value: '7/password'
  </code></pre>
  
  <p>What can i do to fix this?</p>