Not quite sure why this will not raise an error when the number passed in is > or < 16.

Credit Card class

class CreditCard
  def initialize(nums)
      @nums = nums
  end

  def check_card
  raise ArgumentError unless @nums.length == 16
  s1 = s2 = 0
  @nums.to_s.reverse.chars.each_slice(2) do |odd, even| 
    s1 += odd.to_i
 
    double = even.to_i * 2
    double -= 9 if double >= 10
    s2 += double
  end
  (s1 + s2) % 10 == 0
end

end