IO.fdwalk strangeness with ruby 1.9.3

fdwalk.rb

require 'ffi'

class IO
  extend FFI::Library
  ffi_lib FFI::Library::LIBC

  attach_function :fdwalk_c, :fdwalk, [:pointer, :pointer], :void

  def self.fdwalk(lowfd)
    func = FFI::Function.new(:int, [:pointer, :int]){ |cd, fd|
      yield File.new(fd) if fd >= lowfd
    }

    ptr  = FFI::MemoryPointer.new(:int)
    ptr.write_int(lowfd)

    fdwalk_c(func, ptr)
  end
end

#IO.fdwalk(0){ |f| p f.fileno } # Works
IO.fdwalk(0){ } # fd error