diff --git a/core/warning.js b/core/warning.js
index e08088e94..6ed5d0c02 100644
--- a/core/warning.js
+++ b/core/warning.js
@@ -149,7 +149,7 @@ Warning.prototype.setText = function(text, id) {
  */
 Warning.prototype.getText = function() {
   const allWarnings = [];
-  for (let id in this.text_) {
+  for (const id in this.text_) {
     allWarnings.push(this.text_[id]);
   }
   return allWarnings.join('\n');