Why do I get an error installing the JSON gem in Ubuntu?

Using Ubuntu, when I run "`bundle install`" to set up my Rails environment, it throws an error during the JSON gem installation: ``` Installing json (1.7.3) with native extensions Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension. /usr/bin/ruby1.9.1 extconf.rb /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError) from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require' from extconf.rb:1:in `<main>' Gem files will remain installed in /home/danny/.bundler/tmp/2040/gems/json-1.7.3 for inspection. Results logged to /home/danny/.bundler/tmp/2040/gems/json-1.7.3/ext/json/ext/parser/gem_make.out An error occured while installing json (1.7.3), and Bundler cannot continue. Make sure that `gem install json -v '1.7.3'` succeeds before bundling. ``` When I try installing only the JSON 1.7.3 gem it also gives me an error: ``` Building native extensions. This could take a while... ERROR: Error installing json: ERROR: Failed to build gem native extension. /usr/bin/ruby1.9.1 extconf.rb /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError) from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require' from extconf.rb:1:in `<main>' Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.7.3 for inspection. Results logged to /var/lib/gems/1.9.1/gems/json-1.7.3/ext/json/ext/parser/gem_make.out ``` I've installed RVM and am running Ruby 1.9.3; Why can't it install the JSON gem?