--- a/toolkit/content/widgets/notification.xml
+++ b/toolkit/content/widgets/notification.xml
@@ -117,17 +117,18 @@
               // added to the first button (unless that button has isDefault
               // set to false). There cannot be multiple default buttons.
               var defaultElem;
 
               for (var b = 0; b < aButtons.length; b++) {
                 var button = aButtons[b];
                 var buttonElem = document.createElementNS(XULNS, "button");
                 buttonElem.setAttribute("label", button.label);
-                buttonElem.setAttribute("accesskey", button.accessKey);
+                if (typeof button.accessKey == "string")
+                  buttonElem.setAttribute("accesskey", button.accessKey);
                 if (typeof button.type == "string") {
                   buttonElem.setAttribute("type", button.type);
                   if ((button.type == "menu-button" || button.type == "menu") &&
                       "popup" in button) {
                     buttonElem.appendChild(button.popup);
                     delete button.popup;
                   }
                   if (typeof button.anchor == "string")