diff --git a/ext/psych/psych.gemspec b/ext/psych/psych.gemspec
index e9e36e6633..291d8345b4 100644
--- a/ext/psych/psych.gemspec
+++ b/ext/psych/psych.gemspec
@@ -64,4 +64,6 @@ DESCRIPTION
   else
     s.extensions = ["ext/psych/extconf.rb"]
   end
+
+  s.add_dependency 'stringio'
 end