diff --git a/Gemfile b/Gemfile
index 66575a7e..12d04d4a 100644
--- a/Gemfile
+++ b/Gemfile
@@ -75,7 +75,7 @@ group :jekyll_optional_dependencies do
   gem "kramdown-syntax-coderay"
   gem "matrix"
   gem "mime-types", "~> 3.0"
-  gem "rdoc", "~> 6.3.0"
+  gem "rdoc", "~> 6.0"
   gem "tomlrb"
 
   platforms :ruby, :mswin, :mingw, :x64_mingw do