Unable to install gem - Failed to build gem native extension - cannot load such file -- mkmf (LoadError)

Ruby 1.9.3 The part of Gemfile #............... gem "pony" gem "bcrypt-ruby", :require => "bcrypt" gem "nokogiri" #.................. When I'm trying to install gems, I get an error alex@ubuntu:~/$ bundle Fetching gem metadata from http://rubygems.org/......... Fetching gem metadata from http://rubygems.org/.. Enter your password to install the bundled RubyGems to your system: #####............................................................ Installing bcrypt-ruby (3.0.1) 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:36:in `<main>' Gem files will remain installed in /home/alex/.bundler/tmp/5526/gems/bcrypt-ruby-3.0.1 for inspection. Results logged to /home/alex/.bundler/tmp/5526/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out An error occurred while installing bcrypt-ruby (3.0.1), and Bundler cannot continue. Make sure that `gem install bcrypt-ruby -v '3.0.1'` succeeds before bundling. Then I'm doing this sudo gem install bcrypt-ruby -v '3.0.1' Building native extensions. This could take a while... ERROR: Error installing bcrypt-ruby: 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:36:in `<main>' Gem files will remain installed in /var/lib/gems/1.9.1/gems/bcrypt-ruby-3.0.1 for inspection. Results logged to /var/lib/gems/1.9.1/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out and getting an error as well. What did I miss?