diff --git a/ChangeLog b/ChangeLog
index 6360273bfa..3c713064e1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Jun 13 12:23:39 2011  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* ext/psych/lib/psych/deprecated.rb (Object#to_yaml_properties):
+	  undef to_yaml_properties before redefine it.
+
 Mon Jun 13 11:30:10 2011  Martin Bosslet  <Martin.Bosslet@googlemail.com>
 
 	* ext/openssl/ossl_digest.c: allow Digests to be created by sn, ln or
diff --git a/ext/psych/lib/psych/deprecated.rb b/ext/psych/lib/psych/deprecated.rb
index f4034e32c0..333c3a1016 100644
--- a/ext/psych/lib/psych/deprecated.rb
+++ b/ext/psych/lib/psych/deprecated.rb
@@ -76,6 +76,7 @@ module Psych
 end
 
 class Object
+  undef :to_yaml_properties rescue nil
   def to_yaml_properties # :nodoc:
     instance_variables
   end