diff --git a/ChangeLog b/ChangeLog
index 84f9093a14..5f12db81a5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Aug 21 18:54:56 2010  wanabe  <s.wanabe@gmail.com>
+
+	* vm.c (vm_define_method): copy iseq to avoid overwriting iseq->klass.
+	  #2502, #3136. see #2420.
+
 Sat Aug 21 17:36:42 2010  Tanaka Akira  <akr@fsij.org>
 
 	* ext/pathname/pathname.c (path_split): Pathname#split translated from
diff --git a/vm.c b/vm.c
index e62c9a4b7b..eb49722fcd 100644
--- a/vm.c
+++ b/vm.c
@@ -1844,6 +1844,11 @@ vm_define_method(rb_thread_t *th, VALUE obj, ID id, VALUE iseqval,
     rb_iseq_t *miseq;
     GetISeqPtr(iseqval, miseq);
 
+    if (miseq->klass) {
+       iseqval = rb_iseq_clone(iseqval, 0);
+       GetISeqPtr(iseqval, miseq);
+    }
+
     if (NIL_P(klass)) {
 	rb_raise(rb_eTypeError, "no class/module to add method");
     }