JRuby's Encoding.default_external is causing trouble

folder.rb

require 'ffi'

class Windows
  extend FFI::Library
  ffi_lib :shell32

  attach_function :SHGetFolderPath, :SHGetFolderPathW, [:ulong, :int, :ulong, :ulong, :buffer_out], :ulong

  def self.folder
    buf = 0.chr * 1024
    buf.encode!('UTF-16LE')

    if SHGetFolderPath(0, 47, 0, 1, buf) != 0
      raise SystemCallError, FFI.errno, "SHGetFolderPath"
    end

    buf.encode(Encoding.default_external).strip
  end
end

p Windows.folder

# MRI

"C:\\Documents and Settings\\All Users\\Start Menu\\Programs\\Administrative Tools"

# JRuby

Encoding::UndefinedConversionError: Input length = 1
  encode at org/jruby/RubyString.java:7446
  folder at folders.rb:17
  (root) at folders.rb:21

Encoding.default_external # => IBM437 in MRI, Windows-1252 in JRuby