Ripper causes Encoding::CompatibilityError

gistfile1.rb

# coding: utf-8                               
 
text = <<EOS                                  
# coding: utf-8                               
'あ 
あ' 
EOS 
 
require 'ripper'                              
Ripper.new(text).parse                        
 
#  $ ruby -v ripper_encodingerror.rb        
#  ruby 1.9.3dev (2011-09-23 revision 33323) [x86_64-darwin11.1.0]
#  ripper_encodingerror.rb:10:in `parse': incompatible character encodings: ASCII-8BIT and UTF-8 (Encoding::CompatibilityError)
#   from ripper_encodingerror.rb:10:in `<main>'

ちなみに text の magic comment のあるなしで挙動は変わりません