Rails - Jan 2006 - Agile arguement error

Working my way through chapter 8 of "Agile Web Development...",
I''m
getting the following error when clicking the add_to_cart button.

 ArgumentError in Store#add_to_cart
    wrong number of arguments (0 for 1)

What am I doing wrong?  I know it''s got to be some thing stupid.

Request:
Parameters: {"id"=>"1"}

session dump:
---
:cart: !ruby/object:Cart
  items: []
  total_price: 0.0
flash: !ruby/hash:ActionController::Flash::FlashHash {}

and the method def:

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

Hi, yet another n00b working through the Agile book

Previously ted scharff wrote about a "ArgumentError in Store#add_to_cart
    wrong number of arguments (0 for 1)"

I got the same argument, I think by not putting the (product) in the
add_product function in class Cart.

Now, probably through some similar typo, I''m getting a

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}/app/models/cart.rb:12:in `add_product''
#{RAILS_ROOT}/app/controllers/store_controller.rb:10:in `add_to_cart''

I probably just need to go to bed and look at this tomorrow, but if
someone can use their telepathy and see what I''m doing wrong I
wouldn''t mind hearing about it...
I''ve been comparing the source I mostly typed in by hand w/ what the
book shows, but don''t even know enough about Ruby/Rails at this point
to even do the equivalent of Java System.out.println() and see what''s
nil that shouldn''t be....