Heroku Platform API gem — NameError: uninitialized constant ActiveJobClass::Heroku

Question

I'm trying to allow my users to CNAME their custom domain to my Rails app hosted on Heroku. I tried to follow this article but I'm getting this error in my logs when I ran the code:

WARN: NameError: uninitialized constant HerokuDomainJob::Heroku

This is my code in heroku_domain_job.rb

require 'platform-api'

class HerokuDomainJob < ApplicationJob
  queue_as :default

  def perform(domain, action)
    heroku = PlatformAPI.connect_oauth(ENV['HEROKU_API_KEY'])

    begin
      case action
      when "add"
        heroku.domain.create(my_app_name, "hostname" => domain)
      when "remove"
        heroku.domain.delete(my_app_name, domain)
      end

    rescue Heroku::API::Errors::RequestFailed => e
      Rails.logger.error "[Heroku Domain Worker] ERROR: #{e}"
    end
  end

end

This is the code in my User model

class User < ApplicationRecord
  after_save :update_heroku_domains
  ...
  private

  def update_heroku_domains
    if self.domain_changed? && self.domain_was.present?
      HerokuDomainJob.perform_later(self.domain_was, "remove")
    end
    HerokuDomainJob.perform_later(self.domain, "add")
  end

end

Essentially, the code is almost identical as per the article's so I don't really understand where I got it wrong. Appreciate any help.

Thank you.