curr.exdates[i] might not be a date, so toISOString() may fail too

modules/default/calendar/vendor/ical.js/node-ical.js

@@ -43,9 +43,11 @@ ical.objectHandlers['END'] = function(val, params, curr, stack){
       }
     }
     for (var i in curr.exdates) {
+        if( typeof (curr.exdates[i]) === "date") {
             rule += ' EXDATE:' + curr.exdates[i].toISOString().replace(/[-:]/g, '');
             rule = rule.replace(/\.[0-9]{3}/, '');
         }
+    }
     try {
       curr.rrule = rrulestr(rule);
     }