diff --git a/active_model_serializers.gemspec b/active_model_serializers.gemspec
index 3ae91fc1..b39f3bbf 100644
--- a/active_model_serializers.gemspec
+++ b/active_model_serializers.gemspec
@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "activemodel", ">= 4.0"
-
+  spec.add_dependency 'thread_safe','~> 0.3', '>= 0.3.4'
   spec.add_development_dependency "rails", ">= 4.0"
   spec.add_development_dependency "bundler", "~> 1.6"
   spec.add_development_dependency "rake"
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index c4630ab6..ef3f31af 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -1,3 +1,5 @@
+require 'thread_safe'
+
 module ActiveModel
   class Serializer
     extend ActiveSupport::Autoload