diff --git a/static/js/lightbox_canvas.js b/static/js/lightbox_canvas.js
index e291c82d75..9a7b0226e9 100644
--- a/static/js/lightbox_canvas.js
+++ b/static/js/lightbox_canvas.js
@@ -43,7 +43,7 @@ var LightboxCanvas = (function () {
 
             // wheelEvent.deltaMode is a value that describes what the unit is
             // for the `deltaX`, `deltaY`, and `deltaZ` properties.
-            const DELTA_MODE = {
+            var DELTA_MODE = {
                 PIXEL: 0,
                 LINE: 1,
                 PAGE: 2,