diff --git a/lib/chargify_api_ares.rb b/lib/chargify_api_ares.rb
index <HASH>..<HASH> 100644
--- a/lib/chargify_api_ares.rb
+++ b/lib/chargify_api_ares.rb
@@ -1,5 +1,7 @@
 # Chargify API Wrapper using ActiveResource.
 #
+require 'thread'
+
 begin
   require 'active_resource'
 rescue LoadError
Ensure compatibility with ARES <I>.x and latest rubygems