diff --git a/index.js b/index.js
index 165c285..fbbe780 100644
--- a/index.js
+++ b/index.js
@@ -1,7 +1,7 @@
 module.exports = prettierBytes
 
 function prettierBytes (num) {
-  if (typeof num !== 'number' || Number.isNaN(num)) {
+  if (typeof num !== 'number' || isNaN(num)) {
     throw new TypeError('Expected a number, got ' + typeof num)
   }