尝试使用Zookeeper gem时获取“未初始化的常量Zookeeper(NameError)”

我想用 zookeeper gem

require 'rubygems'
require 'zookeeper'
z = Zookeeper.new("localhost:2181")
z.get_children(:path => "/")

导致错误:

./zookeeper.rb:3: uninitialized constant Zookeeper (NameError)
 from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
 from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
 from zookeeper.rb:2

我在搜索和寻找代码,我真的不知道该怎么做。。。