Uninitialized constant Custom::Namespace::Class

[letronje]

I am getting an error of the form => uninitialized constant Custom::Namespace::Klass

..../rails_root/vendor/cache/ruby/1.9.1/gems/rake-0.9.2/lib/rake/ext/module.rb:36:in `const_missing'
......

The app works fine when rack fiber pool and em is disabled.

Here's the config:

Gemfile:

gem 'thin'
gem 'em-synchrony'
gem 'rack-fiber_pool'

config/database.yml

development:
adapter: em_mysql2
........

config/environment.rb

require 'em-synchrony/mysql2'
require 'em-synchrony/activerecord'
require 'rack/fiber_pool'

config/environments/development.rb

config.middleware.insert_before ActionDispatch::Cookies, Rack::FiberPool, :rescue_exception => Proc.new { |env, exception| [503, {}, ([exception.message] + exception.backtrace).join("\n")] }

config.threadsafe!

Any idea what could be wrong ?