If you also get a weird seg fault error while doing gem install, like the following:

localhost:~/Desktop/my_git/myProg birdchan$ gem install bundler
Successfully installed bundler-1.2.2
1 gem installed
/Users/birdchan/.rvm/gems/ruby-1.9.3-p194/gems/json-1.7.5/lib/json/ext/parser.bundle: [BUG] Segmentation fault
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]

It’s likely that your current ruby may be an older one.

localhost:~/Desktop/my_git/myProg birdchan$ ruby -v
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]

Try using rvm to make sure you have a more updated ruby.

localhost:~/Desktop/my_git/myProg birdchan$ rvm use
Using /Users/birdchan/.rvm/gems/ruby-1.9.3-p194

localhost:~/Desktop/my_git/myProg birdchan$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.3.0]

localhost:~/Desktop/my_git/myProg birdchan$ gem install bundler
Successfully installed bundler-1.2.2
1 gem installed
Installing ri documentation for bundler-1.2.2...
Installing RDoc documentation for bundler-1.2.2...