ruby-on-rails – 为什么渲染：view.json.erb中的json导致错误,

def some_action
  @foo = 'bar'
end

在some_action.json.erb中：

<%= render :json => @foo %>

这会导致错误：

ArgumentError - You invoked render but did not give any of :partial,:template,:inline,:file or :text option.:
  (gem) actionpack-3.2.10/lib/action_view/renderer/template_renderer.rb:36:in `ActionView::TemplateRenderer#determine_template'
  (gem) actionpack-3.2.10/lib/action_view/renderer/template_renderer.rb:10:in `ActionView::TemplateRenderer#render'
  (gem) actionpack-3.2.10/lib/action_view/renderer/renderer.rb:36:in `ActionView::Renderer#render_template'
  (gem) actionpack-3.2.10/lib/action_view/renderer/renderer.rb:17:in `ActionView::Renderer#render'
  (gem) actionpack-3.2.10/lib/action_view/helpers/rendering_helper.rb:24:in `#<Class:0xb4a2378>#render'
   ...

我尽可能地绕着它,我看不出为什么渲染：json => ……不应该在这里工作.我知道我可以直接从控制器为format.json渲染,我试图通过一个视图来做,而不是手动制作.erb中的json.

Ruby 1.9上的Rails 3.2.10