diff --git a/Gemfile b/Gemfile
index 3c4cb0ef2a5..27461f0380f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -16,6 +16,7 @@ group :development do
   # documentation
   gem "hoe-markdown", "= 1.4.0"
   gem "rdoc", "= 6.4.0"
+  gem "psych", "~> 4.0" # psych 5 isn't building in places yet https://github.com/ruby/setup-ruby/issues/409
 
   # parser generator
   gem "rexical", "= 1.0.7"