diff --git a/sub-project/Gemfile b/sub-project/Gemfile
index 85f78a68..003be6ec 100644
--- a/sub-project/Gemfile
+++ b/sub-project/Gemfile
@@ -35,5 +35,5 @@ group :test do
   gem 'test-unit'
 end
 group :local do
-  gem 'rdoc'
+  gem 'rdoc', '>= 6.3.4.1'
 end
\ No newline at end of file