RSpec subject behaviour question

Rspec_subject_behaviour_question.rb

require 'spec_helper'

class Foo
  def self.bar(first, second, third)
    err_msg = '%s cannot be nil'
    raise ArgumentError, err_msg % 'first' unless first
    raise ArgumentError, err_msg % 'second' unless second
    raise ArgumentError, err_msg % 'third' unless third

    "#{first} #{second} #{third}"
  end
end

describe Foo do
  [
    { first: nil, second: 2,   third: 3 },
    { first: 1,   second: nil, third: 3 },
    { first: 1,   second: 2,   third: nil }
  ].each_with_index do |args_hash, index|
    subject { Foo.bar(*args_hash.values) }

    it "raises an ArgumentError when #{args_hash.keys[index]} is nil" do
      expected_err_msg = "#{args_hash.keys[index]} cannot be nil"

      # this works:
      expect { Foo.bar(*args_hash.values) }.to raise_error(ArgumentError, expected_err_msg) }

      # this does not:
      # Using subject in this way appears to keep sending through the last set of args
      # each time it is called
      #expect { subject }.to raise_error(ArgumentError, expected_err_msg)
    end
  end
end