diff --git a/active_model_serializers.gemspec b/active_model_serializers.gemspec
index 2ad4ee54..39b8ba2a 100644
--- a/active_model_serializers.gemspec
+++ b/active_model_serializers.gemspec
@@ -21,5 +21,6 @@ Gem::Specification.new do |gem|
   gem.required_ruby_version = ">= 1.9.3"
 
   gem.add_dependency "activemodel", ">= 3.2"
+  gem.add_dependency "concurrent-ruby", "~> 1.0"
   gem.add_development_dependency "rails", ">= 3.2"
 end