diff --git a/calendar/base/content/calendar-item-editing.js b/calendar/base/content/calendar-item-editing.js
index c09926f67e37..ab4f2d296858 100644
--- a/calendar/base/content/calendar-item-editing.js
+++ b/calendar/base/content/calendar-item-editing.js
@@ -180,6 +180,7 @@ function openEventDialog(calendarItem, calendar, mode, callback)
 // whether the user wants to edit all instances, or just this one.  This
 // function prompts this question (if the item is actually an instance of a
 // recurring event) and returns the appropriate item that should be modified.
+// Returns null if the prompt was cancelled.
 function getOccurrenceOrParent(occurrence) {
     // Check if this actually is an instance of a recurring event
     if (occurrence == occurrence.parentItem) {
diff --git a/calendar/base/content/calendar-views.js b/calendar/base/content/calendar-views.js
index a833303b5392..47c59f4ea688 100644
--- a/calendar/base/content/calendar-views.js
+++ b/calendar/base/content/calendar-views.js
@@ -94,6 +94,9 @@ var calendarViewController = {
         var itemToEdit;
         if (aNewStartTime && aNewEndTime) {
             itemToEdit = getOccurrenceOrParent(aOccurrence);
+            if (!itemToEdit) {
+                return;
+            }
             var instance = itemToEdit.clone();
 
             // if we're about to modify the parentItem, we need to account
@@ -143,6 +146,9 @@ var calendarViewController = {
 
     deleteOccurrence: function (aOccurrence) {
         var itemToDelete = getOccurrenceOrParent(aOccurrence);
+        if (!itemToDelete) {
+            return;
+        }
         if (!itemToDelete.parentItem.hasSameIds(itemToDelete)) {
             var event = itemToDelete.parentItem.clone();
             event.recurrenceInfo.removeOccurrenceAt(itemToDelete.recurrenceId);