关于gem install contacts

最近要用到导入联系人的功能.但是gem install contacts却报错.

错误如下:

Building native extensions.  This could take a while...
ERROR:  Error installing contacts:
        ERROR: Failed to build gem native extension.

c:/ruby/bin/ruby.exe extconf.rb install contacts
creating Makefile

nmake
'nmake' 不是内部或外部命令，也不是可运行的程序
或批处理文件。


Gem files will remain installed in c:/ruby/lib/ruby/gems/1.8/gems/json-1.1.2 for
 inspection.
Results logged to c:/ruby/lib/ruby/gems/1.8/gems/json-1.1.2/ext/json/ext/parser/
gem_make.out

想请教为什么会有这样的错误,应该如何解决.谢谢