diff --git a/Gemfile b/Gemfile
index cac1480..98e79da 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,6 +6,6 @@ gem 'rake-jekyll', '~> 1.1.0'
 
 group :jekyll_plugins do
   gem 'jekyll-asciidoc', '~> 3.0.0'
-  gem 'asciidoctor-pdf'
+  gem 'asciidoctor-pdf', '~> 2.3.18'
   gem 'prawn-gmagick'
 end