--- a/calendar/base/content/widgets/calendar-minimonth.js
+++ b/calendar/base/content/widgets/calendar-minimonth.js
@@ -25,17 +25,17 @@
      * @implements {calIObserver}
      * @implements {calICompositeObserver}
      * @implements {nsIObserver}
      */
     class CalendarMinimonth extends MozXULElement {
         constructor() {
             super();
             // Set up custom interfaces.
-            // calIObserver is not used like the others are, so it is not set up here.
+            this.calIObserver = this.getCustomInterfaceCallback(Ci.calIObserver);
             this.calICompositeObserver = this.getCustomInterfaceCallback(Ci.calICompositeObserver);
             this.calIOperationListener = this.getCustomInterfaceCallback(Ci.calIOperationListener);
             this.nsIObserver = this.getCustomInterfaceCallback(Ci.nsIObserver);
         }
 
         static get inheritedAttributes() {
             return {
                 ".minimonth-header": "readonly,month,year",
@@ -964,15 +964,15 @@
             }
 
             // Remove pref observer.
             Services.prefs.getBranch("").removeObserver("calendar.", this.nsIObserver);
         }
     }
 
     MozXULElement.implementCustomInterface(CalendarMinimonth, [
-        // Ci.calIObserver is omitted since it is not used like the others are.
+        Ci.calIObserver,
         Ci.calICompositeObserver,
         Ci.calIOperationListener,
         Ci.nsIObserver
     ]);
     customElements.define("calendar-minimonth", CalendarMinimonth);
 }