diff --git a/tests/test.d/util.rb b/tests/test.d/util.rb
index 277337cfa..5aae085de 100644
--- a/tests/test.d/util.rb
+++ b/tests/test.d/util.rb
@@ -1,3 +1,5 @@
+require 'digest/md5'
+
 $message_mutex = Mutex.new
 def show_message message
   $message_mutex.lock
@@ -35,6 +37,12 @@ class Array
   end
 end
 
+class String
+  def md5
+    Digest::MD5.hexdigest self
+  end
+end
+
 # install md5 handler
 case RUBY_PLATFORM
 when /darwin/