interesting regex behavior on rbx

gistfile1.txt

$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.3.0]
$ pry
main> 'FooBar'.split(/(?=[A-Z])/).map{|_| _.downcase}.join('_')
=> "foo_bar"
main> 'FooBar'.split(/(?=[A-Z])/)
=> ["Foo", "Bar"]

$ ruby -v
rubinius 2.0.0dev (1.9.3 e817532e yyyy-mm-dd JI) [x86_64-apple-darwin11.3.0]
$ rbx -S pry
main> 'FooBar'.split(/(?=[A-Z])/).map{|_| _.downcase}.join('_')
NoMethodError: undefined method `downcase' on nil:NilClass.
from kernel/delta/kernel.rb:81:in `downcase (method_missing)'
main> 'FooBar'.split(/(?=[A-Z])/)
=> ["Foo", "", "", "Bar"]

$ jruby -S pry
Need to `gem install ffi`
main> 'FooBar'.split(/(?=[A-Z])/).map{|_| _.downcase}.join('_')
=> "foo_bar"
main> 'FooBar'.split(/(?=[A-Z])/)
=> ["Foo", "Bar"]