diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index 54ef81aee8..477a42114b 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -2,6 +2,7 @@ require 'abstract_unit'
 require 'active_support/time'
 require 'time_zone_test_helpers'
 require 'active_support/core_ext/string/strip'
+require 'yaml'
 
 class TimeWithZoneTest < ActiveSupport::TestCase
   include TimeZoneTestHelpers