1 files changed, 3 insertions, 2 deletions
diff --git a/activesupport/lib/active_support/core_ext/module/synchronization.rb b/activesupport/lib/active_support/core_ext/module/synchronization.rb
index de76a069d6..ed16c2f71b 100644
--- a/activesupport/lib/active_support/core_ext/module/synchronization.rb
+++ b/activesupport/lib/active_support/core_ext/module/synchronization.rb
@@ -1,11 +1,12 @@
+require 'thread'
 require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/array/extract_options'
 
 class Module
   # Synchronize access around a method, delegating synchronization to a
-  # particular mutex. A mutex (either a Mutex, or any object that responds to 
+  # particular mutex. A mutex (either a Mutex, or any object that responds to
   # #synchronize and yields to a block) must be provided as a final :with option.
-  # The :with option should be a symbol or string, and can represent a method, 
+  # The :with option should be a symbol or string, and can represent a method,
   # constant, or instance or class variable.
   # Example:
   #   class SharedCache