diff --git a/rackstash.gemspec b/rackstash.gemspec
index b612b03..42dc134 100644
--- a/rackstash.gemspec
+++ b/rackstash.gemspec
@@ -30,6 +30,9 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
+  spec.add_dependency 'concurrent-ruby', '~> 1.0', '>= 1.0.2'
+  spec.add_dependency 'ref', '~> 2.0'
+
   spec.add_development_dependency 'bundler', '~> 1.12'
   spec.add_development_dependency 'rake', '~> 10.0'
   spec.add_development_dependency 'rspec', '~> 3.0'