rails 3.1 undefined method rescue_action_locally is making me miserable

I'm stumped.

I inherited a Rails 3.1 app and I'm getting this weird error...

NoMethodError - undefined method `rescue_action_locally' for #<AssignmentsController:0x000000042583d0>:
  app/controllers/application_controller.rb:362:in `process_exception'

And error output is pointing me to line 362 ...

    def process_exception(exception)
      authorize
      set_cashier
      if rescue_as_normal
        ret = rescue_action(exception)
      else
        ret = rescue_action_locally(exception)  <--- but why is this exploding?
      end
      if request and request.xhr?
        response.content_type = Mime::JS
      end
      return ret
    end

I'm on rbenv and my ruby is 1.9.3-p551

If I function-click this in Rubymine I can see this exist inside action_dispatch/middleware/show_exeptions.rb:80 ... So I know the thing is there.

I have no idea why this is exploding. Any clues appreciated.