diff --git a/Base/res/js/Spreadsheet/runtime.js b/Base/res/js/Spreadsheet/runtime.js
index 339675109b..06d636facb 100644
--- a/Base/res/js/Spreadsheet/runtime.js
+++ b/Base/res/js/Spreadsheet/runtime.js
@@ -1,3 +1,5 @@
+"use strict";
+
 // FIXME: Figure out a way to document non-function entities too.
 class Position {
     constructor(column, row, sheet) {