Mutex error while using EM.defer with Ruby 2.0.0

async_loop.sh

#!/bin/bash

failed=0
COUNT=25
for (( i=1; i <= $COUNT; i++ ))
do
  echo "Test # $i:"
  rspec ./async_spec.rb
  if (($?)) ; then 
    failed=$((failed+1))
  fi
   
  sleep 1
done

echo
echo "total:  $COUNT"
echo "failed: $failed"

async_spec.rb

require 'fiber'
require 'eventmachine'
require 'rspec'

module RSpec
  module Core
    class ExampleGroup
      class << self
        alias_method :run_alias, :run

        def run(reporter)
          if EM.reactor_running?
            run_alias reporter
          else
            out = nil
            EM.run do
              Fiber.new {
                out = run_alias reporter
                EM.stop
              }.resume
            end
            out
          end
        end
      end
    end
  end
end

describe "BasicEventMachineTest with Threads" do
  it "should run EventMachine gracefully and schedule callback execution in thread" do
    f = Fiber.current
    operation = proc { res = 1 }
    operation_res = 0
    callback = proc { |proc_res| operation_res = proc_res; f.resume }
    EventMachine.defer operation, callback
    Fiber.yield
    # this code should be executed only after the thread's return
    expect(operation_res).to eq(1)
  end
end

With ruby-2.0.0-p247 async_loop.sh sometime fails with error:

1 example, 0 failures/usr/local/lib/ruby/2.0.0/thread.rb:188:in synchronize': Attempt to unlock a mutex which is not locked (ThreadError) from /usr/local/lib/ruby/2.0.0/thread.rb:188:inblock in pop'
from /usr/local/lib/ruby/2.0.0/thread.rb:187:in handle_interrupt' from /usr/local/lib/ruby/2.0.0/thread.rb:187:inpop'
from /usr/local/lib/ruby/gems/2.0.0/gems/eventmachine-1.0.3/lib/eventmachine.rb:1036:in `block in spawn_threadpool'
...