Minitest got error message about document_root_element

[wuwx]

Rails 4.2.4

require 'test_helper'

class CommentsWidgetTest < Apotomo::TestCase
  has_widgets do |root|
    root << widget(:comments)
  end

  test "display" do
    render_widget :comments
    assert_select "h1"
  end
end

CommentsWidgetTest#test_display:
NotImplementedError: Implementing document_root_element makes assert_select work without needing to specify an element to select from.
    test/widgets/comments_widget_test.rb:10:in `block in <class:CommentsWidgetTest>'