Rails - Feb 2006 - NoMethodError in Agile Web Devleopment Depot Tutorial

I''m working on building the shopping cart in the Depot tutorial in the 
Agile Web Development book. I''m at the part (about page 80) where you 
build the ability to add items to the cart. I''m able to get to 
/store/display_cart/ and it seems items are being added, but if I just 
click on a link to add an item (/store/add_to_cart/2) it gives me this:

NoMethodError in Store#add_to_cart

#{RAILS_ROOT}/app/models/cart.rb:10:in `add_product''
#{RAILS_ROOT}/app/controllers/store_controller.rb:11:in `add_to_cart''

Any ideas? Here are various bits of code:

==============================
#store_controller.rb
class StoreController < ApplicationController

  def index
    @products = Product.salable_items
  end


  def add_to_cart
  	product = Product.find(params[:id])
  	@cart = find_cart
    @cart.add_product(product)
  	redirect_to(:action => ''display_cart'')
  end

  def display_cart
    @cart = find_cart
    @items = @cart.items
  end

  private
  def find_cart
    session[:cart] ||= Cart.new
  end
end

==============================
#cart.rb
class Cart
	attr_reader :items
	attr_reader :total_price
		def initialize
			@items = []
			@total_price = 0.0
		end
		def add_product(product)
		@items << LineItem.for_product(product)
		@total_price += product.price
		end
end

==============================
#line_item.rb
class LineItem < ActiveRecord::Base

  belongs_to :product

  def self.for_product(product)
    item = self.new
    item.quantity = 1
    item.product = product
    item.unit_price = product.price
    item
  end

end

==============================
Anybody got any ideas?

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