caches_page :if is ignored (but caches_action :if works)

I've narrowed this down to the smallest case I can:

In a controller with no before_filters specified in either that controller or its parents, caches_page :if still ignores the :if clause.

e.g.

class PagesController < ApplicationController
  #configure the pages we cache
    caches_page :show, :if => Proc.new {|c| ["home", "test-page"].index(c.request.path_parameters["id"])}

never evaluates the :if - it caches every time. In fact, if you replace the command with nonsense, like

class PagesController < ApplicationController
    caches_page :show, :if => Proc.new {|c| dsaasasdadsasd}

all pages are still cached.

I've commented out all before_filters in this controller and in ApplicationController, no impact. There are some before_filters in libraries but it's not obvious why those would matter.

Replacing caches_page with caches_action works as expected.

Let me know if I can help debug.