ArgumentError with CellTests

application_cell.rb

class ApplicationCell < Cell::Rails

  helper ApplicationHelper
  helper I18nHelper

end

page_cell.rb

class PageCell < ApplicationCell

  cache :footer, :footer_cache_key

  def footer(args)
    @locale = args[:locale]
    render
  end

  def footer_cache_key(args)
    {
      :locale  => args[:locale],
      :version => FooApp.version.to_s
    }
  end

end

page_cell_test.rb

require 'test_helper'

class PageCellTest < Cell::TestCase
  tests PageCell

  should "render footer" do
    invoke(:footer, :locale => I18n.locale)
    assert_select "#footer"
  end

end