diff --git a/src/js/tech/html5.js b/src/js/tech/html5.js
index 2d64cb9680..bc9ae8dfd9 100644
--- a/src/js/tech/html5.js
+++ b/src/js/tech/html5.js
@@ -574,7 +574,7 @@ class Html5 extends Tech {
    */
   supportsFullScreen() {
     if (typeof this.el_.webkitEnterFullScreen === 'function') {
-      const userAgent = window.navigator.userAgent;
+      const userAgent = window.navigator && window.navigator.userAgent || "";
 
       // Seems to be broken in Chromium/Chrome && Safari in Leopard
       if ((/Android/).test(userAgent) || !(/Chrome|Mac OS X 10.5/).test(userAgent)) {
diff --git a/src/js/utils/browser.js b/src/js/utils/browser.js
index 72e52daff0..d181aedc8c 100644
--- a/src/js/utils/browser.js
+++ b/src/js/utils/browser.js
@@ -4,7 +4,7 @@
 import document from 'global/document';
 import window from 'global/window';
 
-const USER_AGENT = window.navigator.userAgent;
+const USER_AGENT = window.navigator && window.navigator.userAgent || "";
 const webkitVersionMap = (/AppleWebKit\/([\d.]+)/i).exec(USER_AGENT);
 const appleWebkitVersion = webkitVersionMap ? parseFloat(webkitVersionMap.pop()) : null;
 
@@ -30,6 +30,7 @@ export const IOS_VERSION = (function() {
   if (match && match[1]) {
     return match[1];
   }
+  return null;
 }());
 
 export const IS_ANDROID = (/Android/i).test(USER_AGENT);