diff --git a/Gemfile b/Gemfile
index 1710ec99..d323e2c5 100644
--- a/Gemfile
+++ b/Gemfile
@@ -16,6 +16,7 @@ group :development do
   # depends on "ostruct" explicitly.
   gem "ostruct"
   gem "rake"
+  gem "rdoc"
 end
 
 group :benchmark do