[web2py] Strange Error: Field does not belong to the table

[Ross Peoples]

I am trying to provide an editor for auth_user using SQLFORM. I also am 
trying to add a checkbox to the bottom of the form to set whether or not the 
user is an administrator. If so, then the user gets added to an 
Administrators group when calling form.accepts().

The strange part is, the code works fine if you check the box, then click 
submit, but if you try to uncheck the box, that's when you get the following 
traceback:

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Traceback (most recent call last):
  File "/media/psf/Python/web2py/gluon/restricted.py", line 192, in restricted
    exec ccode in environment
  File 
"/Users/rosspeoples/Dropbox/Code/Python/web2py/applications/init/controllers/administration.py"
 <http://127.0.0.1:8001/admin/edit/init/controllers/administration.py>, line 
650, in <module>
  File "/media/psf/Python/web2py/gluon/globals.py", line 137, in <lambda>
    self._caller = lambda f: f()
  File "/Users/rosspeoples/Dropbox/Code/Python/web2py/gluon/tools.py", line 
2485, in f
    return action(*a, **b)
  File 
"/Users/rosspeoples/Dropbox/Code/Python/web2py/applications/init/controllers/administration.py"
 <http://127.0.0.1:8001/admin/edit/init/controllers/administration.py>, line 
148, in security
    return update()
  File 
"/Users/rosspeoples/Dropbox/Code/Python/web2py/applications/init/controllers/administration.py"
 <http://127.0.0.1:8001/admin/edit/init/controllers/administration.py>, line 
143, in update
    return user_form(user)
  File 
"/Users/rosspeoples/Dropbox/Code/Python/web2py/applications/init/controllers/administration.py"
 <http://127.0.0.1:8001/admin/edit/init/controllers/administration.py>, line 
120, in user_form
    if form.accepts(request.vars, session):
  File "/media/psf/Python/web2py/gluon/sqlhtml.py", line 1203, in accepts
    self.table._db(self.table._id == self.record.id).update(**fields)
  File "/Users/rosspeoples/Dropbox/Code/Python/web2py/gluon/dal.py", line 5401, 
in update
    fields = self.db[tablename]._listify(update_fields,update=True)
  File "/Users/rosspeoples/Dropbox/Code/Python/web2py/gluon/dal.py", line 4677, 
in _listify
    raise SyntaxError, 'Field %s does not belong to the table' % name
SyntaxError: Field is_administrator does not belong to the table

And here is the controller that is adding the checkbox:

        form = SQLFORM(db.auth_user, user, showid=False)
        form[0].insert(-1, TR((
            TD(LABEL('Is Administrator', 
_for='auth_user_is_administrator')),
            TD(INPUT(_type='checkbox', _name='is_administrator', 
_id='auth_user_is_administrator'))
        )))
        
        form.vars.is_administrator = False
        if user:
            if auth.has_membership(user_id=user.id, role='Administrators'):
                form.vars.is_administrator = True
        
        if form.accepts(request.vars, session):
            admin_group = 
db(db.auth_group.role=='Administrators').select().first()
            db((db.auth_membership.user_id==user.id) & 
(db.auth_membership.group_id==admin_group.id)).delete()
            
            if form.vars.is_administrator:
                db.auth_membership.insert(group_id=admin_group.id, 
user_id=user.id)
                
            return 'OK'
        
        return dict(form=form)

The thing that is driving me nuts is that I am doing exactly what the book 
says to do:

http://web2py.com/book/default/chapter/07#Adding-extra-form-elements-to-SQLFORM

Any thoughts? Thanks.