nginx_status settings not working

I followed the instructions here https://github.com/chef/chef-server/blob/12.3.0/RELEASE_NOTES.md#nginx and got the following error when reconfiguring:

$ sudo chef-server-ctl reconfigure
Starting Chef Client, version 12.5.1
resolving cookbooks for run list: ["private-chef::default"]
Synchronizing Cookbooks:
  - private-chef (0.1.0)
  - yum (3.6.1)
  - enterprise (0.5.1)
  - apt (2.7.0)
  - runit (1.6.0)
  - openssl (4.4.0)
  - chef-sugar (3.1.1)
  - packagecloud (0.0.18)
Compiling Cookbooks...
Recipe: private-chef::default
  * directory[/etc/opscode] action create (up to date)
  * directory[/etc/opscode/logrotate.d] action create (up to date)

  ================================================================================
  Recipe Compile Error in /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/recipes/default.rb
  ================================================================================

  RuntimeError
  ------------
  Error loading file: /etc/opscode/chef-server.rb:52:in `from_file': undefined method `[]=' for nil:NilClass

  Cookbook Trace:
  ---------------
    /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/libraries/private_chef.rb:111:in `rescue in from_file'
    /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/libraries/private_chef.rb:108:in `from_file'
    /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/recipes/config.rb:75:in `from_file'
    /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/recipes/default.rb:38:in `from_file'


  Relevant File Content:
  ----------------------
  /opt/opscode/embedded/cookbooks/cache/cookbooks/private-chef/libraries/private_chef.rb:

  104:        # the reconfigure chef run; we don't particularly care what line in the chef
  105:        # recipes is failing to evaluate the loaded file (in the case of what
  106:        # originally triggered this, private_chef.rb), what we care about is which
  107:        # line in the loaded file is causing the error.
  108:        begin
  109:          self.instance_eval(IO.read(filename), filename, 1)
  110:        rescue
  111>>         raise "Error loading file: #{$!.backtrace[0]}: #{$!.message}"
  112:        end
  113:      end
  114:
  115:      def import_legacy_service_config(old_service_key, new_service_key, keys)
  116:        keys.each do |configkey|
  117:          if PrivateChef[old_service_key].has_key? configkey
  118:            PrivateChef[new_service_key][configkey] ||= PrivateChef[old_service_key][configkey]
  119:            PrivateChef[old_service_key].delete configkey
  120:          end


  Running handlers:
[2015-11-19T22:36:33+00:00] ERROR: Running exception handlers
  Running handlers complete
[2015-11-19T22:36:33+00:00] ERROR: Exception handlers complete
  Chef Client failed. 0 resources updated in 03 seconds
[2015-11-19T22:36:33+00:00] FATAL: Stacktrace dumped to /opt/opscode/embedded/cookbooks/cache/chef-stacktrace.out
[2015-11-19T22:36:33+00:00] FATAL: RuntimeError: Error loading file: /etc/opscode/chef-server.rb:52:in `from_file': undefined method `[]=' for nil:NilClass
$

The chef-server.rb file has the following snippet with line number prepended to the line:

     51 nginx['enable_stub_status'] = true
     52 nginx['stub_status']['listen_host'] = "127.0.0.1"
     53 nginx['stub_status']['listen_port'] = "9999"
     54 nginx['stub_status']['location'] = "/nginx_status"

What gives? If I add nginx['stub_status'] = {} before line 52 I can reconfigure fine. But when I browse to https://my_awesome_chef_server_url:9999/nginx_status I get a 404.

I've also tried to reconfigure opscode-manage-ctl and to restart both chef-server-ctl and opscode-manage-ctl.

What gives?

Also, why are these attributes not documented here: https://docs.chef.io/config_rb_server.html ?

Start with RELEASE_NOTES.md#nginx, the config_rb_server documentation, and the nginx settings in /etc/opscode/chef-server.rb. Trace how private_chef.rb loads that configuration and how the nginx_status attributes are consumed, then verify the fix with chef-server-ctl reconfigure and the /nginx_status endpoint.