Rack::Session:Cookie error using Sinatra, Thin, Rails, and Rack::Cascade

I have a combined Sinatra/Rails app that shares a session using Rack::Session::Cookie. The app works fine when started with Rack::Handler::Thin.run app, but if the rackup file is start with thin start, I get an error in Rack::Session::Cookie:

!! Unexpected error while processing request: no marshal_dump is defined for class Proc
no marshal_dump is defined for class Proc
 /usr/local/lib/ruby/gems/1.9.1/gems/rack-1.0.1/lib/rack/session/cookie.rb:64:in `dump'
 /usr/local/lib/ruby/gems/1.9.1/gems/rack-1.0.1/lib/rack/session/cookie.rb:64:in `commit_session'
 /usr/local/lib/ruby/gems/1.9.1/gems/rack-1.0.1/lib/rack/session/cookie.rb:38:in `call'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/connection.rb:76:in `block in pre_process'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/connection.rb:74:in `catch'
 /usr/local/lib/ruby/gems/1.9开发者_如何学Python.1/gems/thin-1.2.5/lib/thin/connection.rb:74:in `pre_process'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/connection.rb:57:in `process'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/connection.rb:42:in `receive_data'
 /usr/local/lib/ruby/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
 /usr/local/lib/ruby/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/backends/base.rb:57:in `start'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/server.rb:156:in `start'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/controllers/controller.rb:80:in `start'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/runner.rb:177:in `run_command'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/lib/thin/runner.rb:143:in `run!'
 /usr/local/lib/ruby/gems/1.9.1/gems/thin-1.2.5/bin/thin:6:in `'

The rackup file looks like this:

require ::File::dirname(__FILE__) + '/config/environment'
require 'thin'

app = Rack::Builder.new {
  use Rails::Rack::Static
  run Rack::Cascade.new([Sinatra::Application, ActionController::Dispatcher.new])
}.to_app

use Rack::Session::Cookie, :key => '_example', :domain => 'example.org',
  :secret => 'secret'

# have to use this
Rack::Handler::Thin.run app, :Port => 4000, :Host => "0.0.0.0"
# want to use: run app

Have you tried something like this:

app = Rack::Builder.new {
  use Rack::Session::Cookie, :key => '_example', :domain => 'example.org', :secret => 'secret'
  use Rails::Rack::Static
  run Rack::Cascade.new([Sinatra::Application, ActionController::Dispatcher.new])
}.to_app

Looks like your problem is that you're using Rack::Session::Cookie outside of app.