diff --git a/ChangeLog b/ChangeLog
index c698c7da5d..e52e84931c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Jul 29 03:11:59 2009  Koichi Sasada  <ko1@atdot.net>
+
+	* proc.c (rb_method_entry_arity): support not_implemented method.
+	  (I have no idea to test it)
+
 Tue Jul 28 19:36:26 2009  Koichi Sasada  <ko1@atdot.net>
 
 	* proc.c (rb_method_entry_arity): support optimized method (send).
diff --git a/proc.c b/proc.c
index 3b83b17c3b..1d5189a844 100644
--- a/proc.c
+++ b/proc.c
@@ -1487,6 +1487,9 @@ rb_method_entry_arity(const rb_method_entry_t *me)
 	      return -(iseq->argc + 1 + iseq->arg_post_len);
 	  }
       }
+      case VM_METHOD_TYPE_UNDEF:
+      case VM_METHOD_TYPE_NOTIMPLEMENTED:
+	return 0;
       case VM_METHOD_TYPE_OPTIMIZED: {
 	  switch (me->body.optimize_type) {
 	    case OPTIMIZED_METHOD_TYPE_SEND:
@@ -1495,9 +1498,8 @@ rb_method_entry_arity(const rb_method_entry_t *me)
 	      break;
 	  }
       }
-      default:
-	rb_bug("rb_method_entry_arity: invalid method entry type (%d)", me->type);
     }
+    rb_bug("rb_method_entry_arity: invalid method entry type (%d)", me->type);
 }
 
 /*