Rack/Sinatra LoadError: cannot load such file

require "bundler/setup"

Will probably fix your error.

Since you are using Bundler with Sinatra you need to require Bundler for the bundled gems to work. You probably have your gems split between Bundler and your gemset. If you have Sinatra and Haml in your gemset but Pony in your Gemfile you will see a LoadError.