diff --git a/browser.js b/browser.js
index bee8b0b7..f907c708 100644
--- a/browser.js
+++ b/browser.js
@@ -44,9 +44,9 @@ function useColors() {
     (typeof window !== 'undefined' && window.console && (console.firebug || (console.exception && console.table))) ||
     // is firefox >= v31?
     // https://developer.mozilla.org/en-US/docs/Tools/Web_Console#Styling_messages
-    (navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/) && parseInt(RegExp.$1, 10) >= 31) ||
+    (navigator.userAgent && navigator.userAgent.toLowerCase().match(/firefox\/(\d+)/) && parseInt(RegExp.$1, 10) >= 31) ||
     // double check webkit in userAgent just in case we are in a worker
-    (navigator.userAgent.toLowerCase().match(/applewebkit\/(\d+)/));
+    (navigator.userAgent && navigator.userAgent.toLowerCase().match(/applewebkit\/(\d+)/));
 }
 
 /**