Mutex with fiber: segfault under 1.9.2

fiber.rb

m = Mutex.new
i = [0]
a = Fiber.new do 
  t = nil
  m.synchronize { t = i[0] }
  Fiber.yield
  m.synchronize { i[0] = t + 1 }
end
b = Fiber.new do
  t = nil
  m.synchronize { t = i[0] }
  Fiber.yield
  m.synchronize { i[0] = t + 1 }
end
p i # => [0]
a.resume
b.resume
a.resume # 1
b.resume # 1
p i # => [1] # should be [2]