diff --git a/ChangeLog b/ChangeLog
index 7cd75bf441..766b9e8320 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Wed Nov 18 10:12:34 2009  Shugo Maeda  <shugo@ruby-lang.org>
+
+	* vm_method.c (rb_undef): should raise TypeError if klass is nil.
+	  1.instance_eval { undef to_s } causes SEGV before this fix.
+
+	* test/ruby/test_undef.rb: new tests for undef.
+
 Wed Nov 18 08:41:42 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (str_utf8_nth): fixed overrun.  [ruby-core:26787]
diff --git a/vm_method.c b/vm_method.c
index d1c68b58f3..215db50080 100644
--- a/vm_method.c
+++ b/vm_method.c
@@ -574,6 +574,9 @@ rb_undef(VALUE klass, ID id)
 {
     rb_method_entry_t *me;
 
+    if (NIL_P(klass)) {
+	rb_raise(rb_eTypeError, "no class to undef method");
+    }
     if (rb_vm_cbase() == rb_cObject && klass == rb_cObject) {
 	rb_secure(4);
     }