diff --git a/src/util/cast.js b/src/util/cast.js
index 8102f04af..604100ec8 100644
--- a/src/util/cast.js
+++ b/src/util/cast.js
@@ -1,5 +1,13 @@
 const Color = require('../util/color');
 
+/**
+ * Store and possibly polyfill Number.isNaN. Number.isNaN can save time over
+ * self.isNaN by not coercing its input. We need to polyfill it to support
+ * Internet Explorer.
+ * @const
+ */
+const _NumberIsNaN = Number.isNaN || isNaN;
+
 /**
  * @fileoverview
  * Utilities for casting and comparing Scratch data-types.
@@ -25,14 +33,14 @@ class Cast {
         if (typeof value === 'number') {
             // Scratch treats NaN as 0, when needed as a number.
             // E.g., 0 + NaN -> 0.
-            if (isNaN(value)) {
+            if (_NumberIsNaN(value)) {
                 return 0;
             }
             return value;
         }
 
         const n = Number(value);
-        if (isNaN(n)) {
+        if (_NumberIsNaN(n)) {
             // Scratch treats NaN as 0, when needed as a number.
             // E.g., 0 + NaN -> 0.
             return 0;