初めて質問します。初心者です。

bundle exec i18n-tasks health
Forest (en, ja) has 3 keys across 2 locales. On average, values are 6 
characters long, keys have 1.0 segments, a locale has 1 keys.
i18n-tasks: Error scanning app/controllers/application_controller.rb: undefined 
method `to_ast' for false:FalseClass
Did you mean?  to_s

このようなエラーが出てくるのですが、エラー文の意味が分からず、解決できずにおります。

application_controller.rbを見てもto_astに該当する場所がなく、何をしたらいいか分かりません。どうすればよいでしょうか?

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  rescue_from ActiveRecord::RecordNotFound, with: :not_found
  before_action :detect_browser
  before_action :set_locale

  def hello
    text = "PARAMS: #{params}"
    render plain: text
  end

  def not_found
    render 'errors/404', status: 404
  end

  private

  def detect_browser
    case request.user_agent
      when /iPhone/i
        request.variant = :smart
    end
  end

  def set_locale
    I18n.locale = locale
  end

  def locale
    @locale ||= (params[:locale] ||
                 http_accept_language.compatible_language_from(I18n.available_locales) ||
                 I18n.default_locale) 
  end

  def default_url_options
    return {} if params[:locale].blank?
      { locale: locale }
    end
  end

end