Rails inverse of Polymorphic association in concern gives TypeError: can't cast Class

I am trying to use the information in this tutorial to learn concerns with polymorphic associations. I have the following:

concerns/taggable.rb:

module Taggable
  extend ActiveSupport::Concern

  included do
    has_many :taggings, :as => :taggable
    has_many :tags, :through => :taggings
  end

  def tag_list
    tags.map(&:name).join(', ')
  end

  def tag_list=(names)
    self.tags = names.split(',').map do |name|
      Tag.where(name: name.strip).first_or_create!
    end
  end

  module ClassMethods
    def tag_counts
      Tag.select('tags.*, count(taggings.tag_id) as count').joins(:taggings).group('taggings.tag_id')
    end
  end
end

models/article.rb:

class Article < ApplicationRecord
  include Taggable

  def self.tagged_with(name)
    Tag.find_by!(name: name).articles
  end

end

models/tagging.rb:

class Tagging < ApplicationRecord

  belongs_to :tag
  belongs_to :taggable, :polymorphic => true

end

models/tag.rb:

class Tag < ApplicationRecord
  has_many :taggings
  has_many :articles, through: :taggings, source: :taggable, source_type: Article

end

The forward associations work just fine. I can ask:

Article.first.tags
#=> Article Load (0.2ms)  SELECT  "articles".* FROM "articles" ORDER BY "articles"."id" ASC LIMIT ?  [["LIMIT", 1]]
Tag Load (0.3ms)  SELECT  "tags".* FROM "tags" INNER JOIN "taggings" ON "tags"."id" = "taggings"."tag_id" WHERE "taggings"."taggable_id" = ? AND "taggings"."taggable_type" = ? LIMIT ?  [["taggable_id", 1], ["taggable_type", "Article"], ["LIMIT", 11]]
=> #<ActiveRecord::Associations::CollectionProxy [#<Tag id: 1, name: "Superhero", created_at: "2017-11-12 21:06:33", updated_at: "2017-11-12 21:06:33">, #<Tag id: 2, name: "Bat", created_at: "2017-11-12 21:06:33"...

But if I do:

Tag.first.articles
Tag Load (0.3ms)  SELECT  "tags".* FROM "tags" ORDER BY "tags"."id" ASC LIMIT ?  [["LIMIT", 1]]
TypeError: can't cast Class

I've tried several variations on the associations in the tag.rb model but can't seem to find any that works.