Spot the subtle error

gistfile1.rb

class CountrySearch
  # Return the phone country code matching a particular
  # two-letter country code (e.g. 'FR' will get France's
  # country code, which is 33).

  def self.country_code_for_alpha2(code)
    return ArgumentError unless code =~ /^..$/
    return nil unless c = Country[code]
    c.country_code
  end
end

You probably want to raise ArgumentError

@btaitelb: bingo!