diff --git a/ChangeLog b/ChangeLog
index 63b9e30f41..f33415a366 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Jun  5 01:14:02 2015  Koichi Sasada  <ko1@atdot.net>
+
+	* vm_insnhelper.c (vm_defined): show additional messages on rb_bug().
+
 Fri Jun  5 01:06:02 2015  Koichi Sasada  <ko1@atdot.net>
 
 	* vm_method.c (rb_add_method_iseq): use intermediate struct to
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index c90638ed8b..1b8560583b 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -2479,7 +2479,7 @@ vm_defined(rb_thread_t *th, rb_control_frame_t *reg_cfp, rb_num_t op_type, VALUE
 		expr_type = DEFINED_METHOD;
 		break;
 	      default:
-		rb_bug("unreachable");
+		rb_bug("vm_defined: unreachable: %u", (unsigned int)def->flags.visi);
 	    }
 	}
 	else {