undefined method `parent' for nil:NilClass

I'm getting this strange error using Rails 3.0.2.

ActionView::Template::Error (undefined method `parent' for nil:NilClass):
app/controllers/channels_controller.rb:19:in `index'

This is the controller, and line 19 is the respond_with(@channels) block.

Where do I start to search for errors?

class ChannelsController < ApplicationController
  before_filter :set_default_client
  respond_to :html, :xml

  def index
    if params[:cache_set]
      @channels = Channel.active.find_all_by_id(params[:cache_set])
    else
      @channels = Channel.active.find_all_by_id(cookies[:channels].split(','))
    end

    respond_with(@channels)
  end
end

This is the full error:

ActionView::Template::Error (undefined method `parent' for nil:NilClass):
  app/controllers/channels_controller.rb:19:in `index'

Rendered /Users/linus/.rvm/gems/ruby-1.8.7-p330/gems/actionpack-3.0.2/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.3ms)
Rendered /Users/linus/.rvm/gems/ruby-1.8.7-p330/gems/actionpack-3.0.2/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (19.8ms)
Rendered /Users/linus/.rvm/gems/ruby-1.8.7-p330/gems/actionpack-3.0.2/lib/action_dispatch/middleware/te开发者_运维技巧mplates/rescues/template_error.erb within rescues/layout (28.6ms)

I'm using Ruby 1.8.7 with Rails 3.0.2. I've also, just in case, tried Rails 3.0.7 and 3.0.0.

Are you in any case using HAML?

I just bumped into this as well and my colleague (who is not on Stackoverflow yet) found out that it was due to multiline comments in the view template

-#
  = helper_method_1
  = helper_method_2

I solved the problem by changing HAML version from 3.1.x to 3.0.24.

My new Gemfile looks like this.

gem "rails", "3.0.2"
gem "haml", "3.0.24"
gem "compass", "0.10.6"

It looks like HAML-edge has fixed this problem. I expect the next release of HAML after 3.1.1 to resolve this.