app error: uninitialized fiber (FiberError)

config.ru

class Weird
  def call env=nil
    [1].each{ # if you comment out the each block, then it works
      Rainbows.sleep 2
    }
    [200, {}, ["ok\n"]]
  end
end

# bad
run Weird.new

# good
run lambda{ |env| Weird.new.call }

rainbows.rb

worker_processes 1

Rainbows! do
  use :CoolioFiberSpawn
end