diff --git a/activesupport/lib/active_support/core_ext/string/output_safety.rb b/activesupport/lib/active_support/core_ext/string/output_safety.rb
index 5babc297c191e..3ee5bcaab476e 100644
--- a/activesupport/lib/active_support/core_ext/string/output_safety.rb
+++ b/activesupport/lib/active_support/core_ext/string/output_safety.rb
@@ -1,4 +1,5 @@
-require "erb"
+require 'erb'
+require 'active_support/core_ext/object/singleton_class'
 
 class ERB
   module Util