rbx strscan

$ rvm use ruby-1.9.3-p362 
Using /Users/me/.rvm/gems/ruby-1.9.3-p362
$ irb
1.9.3p362 :001 > require 'strscan'
 => true 
1.9.3p362 :002 > scanner = StringScanner.new('cölorfül')
 => #<StringScanner 0/10 @ "c\xC3\xB6lo..."> 
1.9.3p362 :003 > scanner.pos = 9
 => 9 
1.9.3p362 :004 > scanner
 => #<StringScanner 9/10 "...orf\xC3\xBC" @ "l"> 
1.9.3p362 :005 > 
$ rvm use rbx-2.0.0-rc1 
Using /Users/me/.rvm/gems/rbx-2.0.0-rc1
$ irb
rubinius-2.0.0.rc1 :001 > require 'strscan'
 => true 
rubinius-2.0.0.rc1 :002 > scanner = StringScanner.new('cölorfül')
 => #<StringScanner 0/8 @ "cölor..."> 
rubinius-2.0.0.rc1 :003 > scanner.pos = 9
RangeError: index out of range (9)
  from /Users/me/.rvm/rubies/rbx-2.0.0-rc1/lib/strscan.rb:15:in `pos='
  from (irb):3
  from kernel/common/block_environment.rb:75:in `call_on_instance'
	from kernel/common/eval.rb:75:in `eval'
	from kernel/common/kernel19.rb:42:in `loop'
	from kernel/common/throw_catch19.rb:8:in `catch'
	from kernel/common/throw_catch.rb:10:in `register'
	from kernel/common/throw_catch19.rb:7:in `catch'
	from kernel/common/throw_catch19.rb:8:in `catch'
	from kernel/common/throw_catch.rb:10:in `register'
	from kernel/common/throw_catch19.rb:7:in `catch'
	from kernel/delta/codeloader.rb:68:in `load_script'
	from kernel/delta/codeloader.rb:118:in `load_script'
	from kernel/loader.rb:615:in `script'
	from kernel/loader.rb:816:in `main'
rubinius-2.0.0.rc1 :004 >