diff --git a/src/browser-polyfill.js b/src/browser-polyfill.js
index bb477fba..66c95337 100644
--- a/src/browser-polyfill.js
+++ b/src/browser-polyfill.js
@@ -6,7 +6,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 "use strict";
 
-if (typeof browser === "undefined") {
+if (typeof window.browser === "undefined") {
   // Wrapping the bulk of this polyfill in a one-time-use function is a minor
   // optimization for Firefox. Since Spidermonkey does not fully parse the
   // contents of a function until the first time it's called, and since it will