ruby-on-rails-3.1 - Railscast 地理编码缺少 JSON::ParserError 常量

Question

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

    multi_json (1.0.3)
   nifty-generators (0.4.6)

我明白了

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

我一定错过了一些定义这一点的东西。有人有想法吗？rc4