Unexpose always throw exception

class Sticker < Grape::Entity
      expose :id
      expose :thumb do |model, opts|
        model.thumb.url
      end
end

class StickerMess < Sticker
      expose :description, :times
      unexpose :thumb
end

The grape server mount as api of rails

 : rails s

Users/benyu/.rvm/gems/ruby-2.0.0-p598/gems/railties-4.0.0/lib/rails/application/routes_reloader.rb:10:in `rescue in execute_if_updated': Rails::Application::RoutesReloader#execute_if_updated delegated to updater.execute_if_updated, but updater is nil: #<Rails::Application::RoutesReloader:0x007ff1c79eded0 @paths=["/Users/benyu/www/babe/config/routes.rb"], @route_sets=[#ActionDispatch::Routing::RouteSet:0x007ff1c4722fa0]> (RuntimeError)

Start by reproducing the Sticker and StickerMess exposure example in a Rails-mounted Grape server using the reported Ruby and Rails versions. Trace the unexpose :thumb path and the Rails route reloader failure; done means the server starts without the reported exception and unexpose behaves as intended.