Error on upgrade 12.4.3 => 12.5.0: recipe compile error [letsencrypt]

Self-hosted omnibus installation on Debian Stretch

In gitlab.rb only the following related to "let's encrypt" is configured:

letsencrypt['enable'] = true
letsencrypt['contact_emails'] = ['xxx@xxx.xx']

Error:

Installing Cookbook Gems:
Compiling Cookbooks...

================================================================================
Recipe Compile Error in /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb
================================================================================

TypeError
---------
no implicit conversion of nil into String

Cookbook Trace:
---------------
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/letsencrypt/libraries/helper.rb:36:in `exist?'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/letsencrypt/libraries/helper.rb:36:in `add_service_alt_name'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/registry.rb:78:in `parse_registry_external_url'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/libraries/registry.rb:24:in `parse_variables'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/helpers/settings_helper.rb:185:in `block in generate_config'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/helpers/settings_helper.rb:183:in `each'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/helpers/settings_helper.rb:183:in `generate_config'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/config.rb:24:in `from_file'
  /opt/gitlab/embedded/cookbooks/cache/cookbooks/gitlab/recipes/default.rb:26:in `from_file'

Relevant File Content:
----------------------
/opt/gitlab/embedded/cookbooks/cache/cookbooks/letsencrypt/libraries/helper.rb:

 29:      # Adds a service's external URL to the certificate's alt_name list so the
 30:      # generated certificate is applicable to that domain also.
 31:
 32:      return unless Gitlab['letsencrypt']['enable']
 33:
 34:      uri = URI(Gitlab["#{service}_external_url"].to_s)
 35:
 36>>     return if !Gitlab['external_url'] || File.exist?(Gitlab["#{service}_nginx"]["ssl_certificate"])
 37:
 38:      # If the default certficate file is missing, configure as an alt_name
 39:      # of the letsencrypt managed certificate
 40:      Gitlab['letsencrypt']['alt_names'] ||= []
 41:      Gitlab['letsencrypt']['alt_names'] << uri.host
 42:
 43:      external_uri = URI(Gitlab['external_url'])
 44:      Gitlab["#{service}_nginx"]["ssl_certificate"] = "/etc/gitlab/ssl/#{external_uri.host}.crt"
 45:      Gitlab["#{service}_nginx"]["ssl_certificate_key"] = "/etc/gitlab/ssl/#{external_uri.host}.key"

System Info:
------------
chef_version=14.13.11
platform=debian
platform_version=9.11
ruby=ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
program_name=/opt/gitlab/embedded/bin/chef-client
executable=/opt/gitlab/embedded/bin/chef-client

The Admin Area is reporting that 12.5.0 is installed, but a gitlab-ctl reconfigure failes with the error above.