Having problems using Treebeard with my django homebrew pages app...

admin.py

import datetime
from django.contrib import admin
from treebeard.admin import TreeAdmin
from dox.models import Page

#--------------------------------------------------------------------------
#
# Admin models:
#
#--------------------------------------------------------------------------

class PageAdmin(TreeAdmin):
    
    save_on_top = True
    
    def save_form(self, request, form, change):
        if not change:
            form.cleaned_data['created'] = datetime.datetime.now()
        return form.save(commit=False)
    
    list_display = ('title', 'url', 'template_name', 'parent',)
    list_editable = ('url', 'template_name', 'parent',)
    
    search_fields = (
        'url',
        'title',
        'content',
        'template_name',
        'meta_about',
        'meta_notes',
        'head_title',
        'head_title_extra',
        'head_description',
        'head_keywords',
        'head_ads',
        'head_extra',
        'description_short',
        'description_long',
        'footer_extra',
        'ad_page_peel',
        'ad_expanding_billboard',
        'ad_leaderboard',
        'ad_leaderboard_bottom',
        'ad_medium_rectangle',
        'ad_half_page',
        'ad_skyscraper',
        'ad_column_block',
        'ad_column_block_bottom',
        'ad_site_skin',
    )
    
    fieldsets = [
        
        ('Page', {
            'fields': [
                'url',
                'template_name',
                'parent',
            ],
        }),
        
        ('Head', {
            'fields': [
                'head_title',
                'head_title_extra',
                'head_description',
                'head_keywords',
                'head_ads',
                'head_extra',
            ], 'classes': ['collapse'],
        }),
        
        ('Body', {
            'fields': [
                'title',
                'description_short',
                'description_long',
                'content',
            ],
        }),
        
        ('Ads', {
            'fields': [
                'ad_page_peel',
                'ad_expanding_billboard',
                'ad_leaderboard',
                'ad_leaderboard_bottom',
                'ad_medium_rectangle',
                'ad_half_page',
                'ad_skyscraper',
                'ad_column_block',
                'ad_column_block_bottom',
                'ad_site_skin',
            ], 'classes': ['collapse'],
        }),
        
        ('Meta', {
            'fields': [
                'meta_about',
                'meta_notes',
            ], 'classes': ['collapse'],
        }),
        
    ]
    

#--------------------------------------------------------------------------
#
# Admin model registers:
#
#--------------------------------------------------------------------------

admin.site.register(Page, PageAdmin)

models.py

import datetime
from django.db import models
from django.utils.translation import ugettext_lazy as _
from treebeard.ns_tree import NS_Node

# http://docs.djangoproject.com/en/dev/internals/contributing/#model-style
# http://code.tabo.pe/django-treebeard/src/93b579395a9c/tbexample

class Page(NS_Node):
    
    """
    created  = models.DateTimeField(_(u'Created'), editable=False)
    modified = models.DateTimeField(_(u'Modified'), editable=False)
    """
    
    created = models.DateTimeField(editable=False)
    
    #----------------------------------
    # Treebeard:
    #----------------------------------
    
    parent = models.ForeignKey('self', blank=True, null=True, db_index=True, related_name='children')
    
    #----------------------------------
    # FlatPage fields:
    #----------------------------------
    
    # https://code.djangoproject.com/browser/django/trunk/django/contrib/flatpages/models.py
    
    url = models.CharField(_('URL'), max_length=100, db_index=True)
    title = models.CharField(_('title'), max_length=200)
    content = models.TextField(_('content'), blank=True)
#     enable_comments = models.BooleanField(_('enable comments'))
    template_name = models.CharField(_('template name'), max_length=70, blank=True, help_text=_("Example: 'files/contact_page.html'. If this isn't provided, the system will use 'files/default.html'."))
    
    #----------------------------------
    # Head:
    #----------------------------------
    
    meta_about = models.TextField(_(u'About'), blank=True)
    meta_notes = models.TextField(_(u'Notes'), blank=True)
    head_title = models.CharField(_(u'Title'), max_length=255, blank=True)
    head_title_extra = models.CharField(_(u'Title extra'), max_length=255, blank=True)
    head_description = models.CharField(_(u'Description'), max_length=255, blank=True)
    head_keywords = models.CharField(_(u'Keywords'), max_length=255, blank=True)
    head_ads = models.TextField(_(u'Ads'), blank=True)
    head_extra = models.TextField(_(u'Extra'), blank=True)
    description_short = models.CharField(_(u'Short description'), max_length=255, blank=True)
    description_long = models.TextField(_(u'Long description'), blank=True)
    
    #----------------------------------
    # Body:
    #----------------------------------
    
    footer_extra = models.TextField(_(u'Footer'), blank=True)
    ad_page_peel = models.CharField(_(u'Page Peel'), max_length=255, blank=True)
    ad_expanding_billboard = models.CharField(_(u'Expanding Billboard'), max_length=255, blank=True)
    ad_leaderboard = models.CharField(_(u'Leaderboard'), max_length=255, blank=True)
    ad_leaderboard_bottom = models.CharField(_(u'Leaderboard • bottom'), max_length=255, blank=True)
    ad_medium_rectangle = models.CharField(_(u'Medium Rectangle'), max_length=255, blank=True)
    ad_half_page = models.CharField(_(u'Half Page'), max_length=255, blank=True)
    ad_skyscraper = models.CharField(_(u'Skyscraper'), max_length=255, blank=True)
    ad_column_block = models.CharField(_(u'Column Block'), max_length=255, blank=True)
    ad_column_block_bottom = models.CharField(_(u'Column Block • bottom'), max_length=255, blank=True)
    ad_site_skin = models.CharField(_(u'Site Skin'), max_length=255, blank=True)
    
    node_order_by = ['created']
    
    class Meta:
        verbose_name = _('dox')
        verbose_name_plural = _('dox\'s')
        ordering = ['tree_id', 'lft']
    
    """
    def save(self, *args, **kwargs):
        if not self.pk:
            self.created = datetime.datetime.now()
        self.modified = datetime.datetime.now()
        super(Page, self).save(*args, **kwargs)
    
    @property
    def is_modified(self):
        if self.modified.date() > self.created.date():
            return True
        return False
    """
    
    @models.permalink
    def get_absolute_url(self):
        return self.url
    
    def __unicode__(self):
        return u"%s -- %s" % (self.url, self.title)

urls.py

from django.conf.urls.defaults import *

urlpatterns = patterns('dox.views',
    
    (r'^(?P<url>.*)$', 'page'),
    
)

views.py

from django import shortcuts, template
from django.template import loader, RequestContext
from django.shortcuts import get_object_or_404
from django.http import Http404, HttpResponse, HttpResponsePermanentRedirect
from django.conf import settings
from django.core.xheaders import populate_xheaders
from django.utils.safestring import mark_safe
from django.views.decorators.csrf import csrf_protect
from dox.models import Page

# Copied from:
# https://code.djangoproject.com/browser/django/trunk/django/contrib/flatpages/views.py

DEFAULT_TEMPLATE = 'files/default.html'

def page(request, url):
    
    if not url.startswith('/'):
        url = '/' + url
    try:
        f = get_object_or_404(Page, url__exact=url)
    except Http404:
        if not url.endswith('/') and settings.APPEND_SLASH:
            url += '/'
            f = get_object_or_404(Page, url__exact=url)
            return HttpResponsePermanentRedirect('%s/' % request.path)
        else:
            raise
    return render_page(request, f)

@csrf_protect
def render_page(request, f):
    
    if f.template_name:
        t = loader.select_template((f.template_name, DEFAULT_TEMPLATE))
    else:
        t = loader.get_template(DEFAULT_TEMPLATE)
    
    f.title = mark_safe(f.title)
    f.content = mark_safe(f.content)
    
    c = RequestContext(request, {
        'page': f,
    })
    response = HttpResponse(t.render(c))
    populate_xheaders(request, response, File, f.id)
    return response

I keep getting:

ImproperlyConfigured at /admin/dox/page/

'PageAdmin.fieldsets[0][1]['fields']' refers to field 'url' that is missing from the form.

When I remove "fieldsets" from the admin, everything works great.

What am I missing here?