diff --git a/mozilla/calendar/base/src/calAlarmService.js b/mozilla/calendar/base/src/calAlarmService.js
index c6146ee7443..a98e079a018 100644
--- a/mozilla/calendar/base/src/calAlarmService.js
+++ b/mozilla/calendar/base/src/calAlarmService.js
@@ -117,7 +117,7 @@ calAlarmServiceClassInfo = {
         var ifaces = [
             Components.interfaces.nsISupports,
             Components.interfaces.calIAlarmService,
-            Components.interfaces.calIObserver
+            Components.interfaces.nsIObserver
         ];
         count.value = ifaces.length;
         return ifaces;