Can't convert nil to String building ActiveSupport RDoc in 1.9 mode

system ~/projects/jruby2 $ jruby --1.9 -S gem install --rdoc --ri activesupport
Fetching: multi_json-1.0.4.gem (100%)
Fetching: activesupport-3.1.3.gem (100%)
Successfully installed multi_json-1.0.4
Successfully installed activesupport-3.1.3
2 gems installed
Installing ri documentation for multi_json-1.0.4...

RDoc::Parser::Ruby failure around line 1 of
lib/multi_json/engines/json_common.rb

Before reporting this, could you check that the file you're documenting
has proper syntax:

  /Users/headius/projects/jruby2/bin/jruby -c lib/multi_json/engines/json_common.rb

RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.

The internal error was:

	(TypeError) can't convert nil into String

ERROR:  While generating documentation for multi_json-1.0.4
... MESSAGE:   can't convert nil into String
... RDOC args: --ri --op /Users/headius/projects/jruby2/lib/ruby/gems/1.8/doc/multi_json-1.0.4/ri --charset=UTF-8 lib LICENSE.md README.md --title multi_json-1.0.4 Documentation --quiet