Possible bug in mutant

mutant-bug.rb

require 'concord'

class Request
  include Concord.new(:input)
end

class Response
  include Concord.new(:input, :output)

  def to_request(new_input = output)
    Request.new(new_input)
  end
end

require 'rspec'

describe Response, '#to_request' do

  let(:object) { Response.new(input, output) }
  let(:input)  { double('input') }
  let(:output) { double('output') }

  let(:expected) { Request.new(new_input) }

  # How come mutant thinks this spec kills all mutations?
  context 'when no new input is given' do
    subject { object.to_request }

    let(:new_input) { output }

    it { should eql(expected) }
  end

  # I think the following context is also needed
  # context 'when new input is given' do
  #   subject { object.to_request(new_input) }

  #   let(:new_input) { double('new_input') }

  #   it { should eql(expected) }
  # end
end