acts_as_list error - help!

I'm having a problem with acts_as_list, and I'm stumped. I have two
models: page and item. A page contains a list of items:

class Page < ActiveRecord::Base
  has_many :items, :dependent => :destroy, :order => :position
  belongs_to :portfolio
...

class Item < ActiveRecord::Base
  belongs_to :page
  acts_as_list :scope => :page
...

When I try to destroy an item, I get this error:

NoMethodError in ItemsController#destroy
undefined method `updated?' for #<Page:0x26eeedc>

If I remove the :dependent => :destroy in the has_many declaration,
everything works fine (the item's page_id is nulled, but the item
isn't destroyed). Any idea what I'm doing wrong? I've Googled til my
fingers bled, but no luck. Any help is greatly appreciated!