diff --git a/src/Std/Data/Stack.lean b/src/Std/Data/Stack.lean
index 7a8c0d7c22..3fc75b1023 100644
--- a/src/Std/Data/Stack.lean
+++ b/src/Std/Data/Stack.lean
@@ -1,3 +1,4 @@
+#lang lean4
 /-
 Copyright (c) 2019 Microsoft Corporation. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.