diff --git a/lib/core/constants.js b/lib/core/constants.js
index 0f827cc4ae0..6ec770dd533 100644
--- a/lib/core/constants.js
+++ b/lib/core/constants.js
@@ -1,3 +1,5 @@
+'use strict'
+
 /** @type {Record<string, string | undefined>} */
 const headerNameLowerCasedRecord = {}