Railscast地理编码缺少JSON :: ParserError常量

Question

在http://railscasts.com/episodes/273-geocoder处从Railscast＃273复制地理编码示例 在Rails 3.1rc4下使用：

    multi_json (1.0.3)
   nifty-generators (0.4.6)

我得到了

NameError in LocationsController#create

uninitialized constant JSON::ParserError
Rails.root: ...

Application Trace | Framework Trace | Full Trace
app/controllers/locations_controller.rb:20:in `create'
This error occurred while loading the following files:
   multi_json/engines/json_gem

我必须遗漏一些定义它的东西。有人有想法吗？rc4