ruby-on-rails - シェフのテンプレート変数 @node がエラーをスローするようです

Question

私は peepcode の Chef チュートリアルを進めています。テンプレート変数を使用すると、何らかの理由でエラーが発生します。例は nginx の場合です。

nginx/attributes/nginx.rb には次のものがあります。

default[:nginx][:worker_processes] = 4

nginx.conf.erb テンプレートでは、次を参照しています。

worker_processes  <%= @node[:nginx][:worker_processes] %>;

以下は、chef-solo を実行すると発生するエラーです。

Template Context:
-----------------
on line #2
  1: user www-data;
  2: worker_processes  <%= @node[:nginx][:worker_processes] %>;
  3: 
  4: error_log  /var/log/nginx/error.log;
  5: pid        /var/run/nginx.pid;


[2013-07-14T19:46:36+02:00] ERROR: Running exception handlers
[2013-07-14T19:46:36+02:00] ERROR: Exception handlers complete
Chef Client failed. 0 resources updated
[2013-07-14T19:46:36+02:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
[2013-07-14T19:46:36+02:00] FATAL: Chef::Mixin::Template::TemplateError: undefined method `[]' for nil:NilClass

エラー出力の他の部分:

Starting Chef Client, version 11.4.4
Compiling Cookbooks...
Converging 3 resources
Recipe: nginx::default
  * package[nginx] action install (up to date)
  * service[nginx] action enable (up to date)
  * service[nginx] action start (up to date)
  * template[/etc/nginx/nginx.conf] action create
================================================================================
Error executing action `create` on resource 'template[/etc/nginx/nginx.conf]'
================================================================================


Chef::Mixin::Template::TemplateError
------------------------------------
undefined method `[]' for nil:NilClass


Resource Declaration:
---------------------
# In /cookbooks/nginx/recipes/default.rb

  8: template "/etc/nginx/nginx.conf" do
  9:    notifies :reload, "service[nginx]"
 10: end



Compiled Resource:
------------------
# Declared in /cookbooks/nginx/recipes/default.rb:8:in `from_file'

template("/etc/nginx/nginx.conf") do
  provider Chef::Provider::Template
  action "create"
  retries 0
  retry_delay 2
  path "/etc/nginx/nginx.conf"
  backup 5
  source "nginx.conf.erb"
  cookbook_name :nginx
  recipe_name "default"
end