i18n issue

gistfile1.rb

class Order < ActiveRecord::Base
  has_many :order_items

  validates_length_of :order_items, :minimum => 1, :message => :empty
  accepts_nested_attributes_for :order_items
end

#en.yml
  activerecord:
    models:
      order:
        order_items:
          quantity: quantidade


How to translate order_items (i.e. "Order items is empty.")?