error on instaling webrat for my rails app

hai all,

    i am getting an error when i am iinstalling webrat for my rails app.i want to use in my rails app cucumber, rspec,webrat to my app test.so please give me some solution to this error.

ERROR: Error installing webrat:
ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0 for inspection. Results logged to /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0/ext/nokogiri/gem_make.out