ohai platform broken on mac osx when using ruby 1.9.2

When using ruby 1.9.2p0 (installed using rvm)

➜ ~ ohai platform
/Users/frank/.rvm/gems/ruby-1.9.2-p0/gems/ohai-0.5.8/lib/ohai/system.rb:231:in `attributes_print': undefined method `to_a' for "mac_os_x":String (NoMethodError)

this works perfectly on the pre-installed 1.8.7:

➜ ~ rvm system
➜ ~ ohai platform
[
"mac_os_x"
]
➜ ~ ruby -v
ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]