NoMethodError in StoreController#add_to_cart

I am building the Depot app from Agile. And an hoping someone can
point out the obvious that I cannot see.

** Here is the error - it occurs when I choose the "add to cart" link.

NoMethodError in StoreController#add_to_cart

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occured while evaluating nil.<<

RAILS_ROOT: ./script/../config/..

#{RAILS_ROOT}/app/models/cart.rb:11:in `add_product'
#{RAILS_ROOT}/app/controllers/store_controller.rb:15:in `add_to_cart'

** controllers/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
     @item = @cart.items
  end

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

end

** models/cart.rb

class Cart
   attr_reader :items
   attr_reader :total_price

   def intialize
     @items = []
     @total_price = 0.0
   end

   def add_product(product)
      @items << LineItem.for_product(product)
      @total_price += product.price
   end
end

** models/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.

Thanks
-Stephen

Fred - THANKS! I have been looking at this for too many hours.