--- a/browser/base/content/tab-content.js
+++ b/browser/base/content/tab-content.js
@@ -329,17 +329,17 @@ var AboutReaderListener = {
   /**
    * NB: this function will update the state of the reader button asynchronously
    * after the next mozAfterPaint call (assuming reader mode is enabled and
    * this is a suitable document). Calling it on things which won't be
    * painted is not going to work.
    */
   updateReaderButton: function(forceNonArticle) {
     if (!ReaderMode.isEnabledForParseOnLoad || this.isAboutReader ||
-        !(content.document instanceof content.HTMLDocument) ||
+        !content || !(content.document instanceof content.HTMLDocument) ||
         content.document.mozSyntheticDocument) {
       return;
     }
 
     this.scheduleReadabilityCheckPostPaint(forceNonArticle);
   },
 
   cancelPotentialPendingReadabilityCheck: function() {