diff --git a/Gemfile b/Gemfile
index 13bee90d..6df880fd 100644
--- a/Gemfile
+++ b/Gemfile
@@ -59,6 +59,7 @@ if RUBY_ENGINE == "ruby"
   gem 'sass'
   gem 'reel-rack'
   gem 'celluloid', '~> 0.16.0'
+  gem 'commonmarker', '~> 0.20.0'
   gem 'simplecov', require: false
 end