diff --git a/Gemfile b/Gemfile
index cfadb0dc8..8cc1bce2b 100644
--- a/Gemfile
+++ b/Gemfile
@@ -16,7 +16,7 @@ gem "rails-controller-testing", "~> 1.0"
 gem "rails-i18n", ">= 4.0.4"
 gem "rails_translation_manager", "~> 0.1.0"
 gem "sass-rails", "~> 5.0"
-gem "slimmer", "~> 13.3"
+gem "slimmer", "~> 13.4"
 gem "uglifier", ">= 1.3.0"
 
 group :development, :test do
diff --git a/Gemfile.lock b/Gemfile.lock
index f822bcb0f..dec635a75 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -302,7 +302,7 @@ GEM
       rubyzip (>= 1.2.2)
     sentry-raven (3.0.0)
       faraday (>= 1.0)
-    slimmer (13.3.0)
+    slimmer (13.4.0)
       activesupport
       json
       nokogiri (~> 1.7)
@@ -387,7 +387,7 @@ DEPENDENCIES
   rubocop-govuk (~> 3)
   sass-rails (~> 5.0)
   scss_lint-govuk (~> 0.2)
-  slimmer (~> 13.3)
+  slimmer (~> 13.4)
   uglifier (>= 1.3.0)
   webmock (~> 3.8.3)
   wraith (~> 4.2)