Ruby error with CSV and << - ruby

Question

Well, I think the code speaks out for itself :)
# book_in_stock.rb
class BookinStock
attr_reader :isbn, :price
def initialize(isbn, price)
#isbn = isbn
#price = Float(price)
end
end
# csv_reader.rb
require 'csv'
class CsvReader
def initialize
#book_in_stock = []
end
def read_in_csv_data(csv_file_name)
CSV.foreach(csv_file_name, headers: true) do |row|
#books_in_stock << BookinStock.new(row["ISBN"], row["Amount"])
end
end
# later we'll see how to use inject to sum a collection
def total_value_in_stock
sum = 0.0
#books_in_stock.each { |book| sum += book.price }
sum
end
def number_of_each_isbn
# ...
end
end
# stock_stats.rb
reader = CsvReader.new()
ARGV.each do |csv_file_name|
STDERR.puts "[+] Processing #{csv_file_name}"
reader.read_in_csv_data(csv_file_name)
end
puts "[+] Total value = #{reader.total_value_in_stock}"
When running I get:
# +search/pickaxe/csv $ ruby1.9.1 test.rb data.csv
# [+] Processing data.csv
# test.rb:23:in `block in read_in_csv_data': undefined method `<<' for nil:NilCla
# ss (NoMethodError)
# from /usr/lib/ruby/1.9.1/csv.rb:1760:in `each'
# from /usr/lib/ruby/1.9.1/csv.rb:1196:in `block in foreach'
# from /usr/lib/ruby/1.9.1/csv.rb:1334:in `open'
# from /usr/lib/ruby/1.9.1/csv.rb:1195:in `foreach'
# from test.rb:22:in `read_in_csv_data'
# from test.rb:46:in `block in <main>'
# from test.rb:44:in `each'
# from test.rb:44:in `<main>'
What've I done wrong?

Answer 1

'#books_in_stock' != '#book_in_stock' # !
(typo)

Answer 2

You have a typo in the initialize function (#book_in_store insteadn of #books_in_store). Is that it?
By the way, the sum of the prices can be done with inject in one line
#books_in_stock.inject(0) { |sum, book| sum + book.price }

Answer 3

If you run the command with ruby -w, it'll turn on warnings, and the interpreter will explain what went wrong:
$ ruby -w book_in_stock.rb
book_in_stock.rb:28: warning: instance variable #books_in_stock not initialized
book_in_stock.rb:28:in `total_value_in_stock': undefined method `each' for nil:NilClass (NoMethodError)
from book_in_stock.rb:47:in `<main>'