diff --git a/lib/json-schema/validator.rb b/lib/json-schema/validator.rb
index a11919b0..af9dc316 100644
--- a/lib/json-schema/validator.rb
+++ b/lib/json-schema/validator.rb
@@ -5,6 +5,7 @@
 require 'date'
 require 'thread'
 require 'timeout'
+require 'stringio'
 require 'yaml'
 
 require 'json-schema/schema/reader'