--- a/toolkit/modules/addons/WebRequest.jsm
+++ b/toolkit/modules/addons/WebRequest.jsm
@@ -634,16 +634,21 @@ HttpObserverManager = {
       }
     }
     delete this.activityErrorsMap;
     this.activityErrorsMap = map;
     return this.activityErrorsMap;
   },
   GOOD_LAST_ACTIVITY: nsIHttpActivityObserver.ACTIVITY_SUBTYPE_RESPONSE_HEADER,
   observeActivity(nativeChannel, activityType, activitySubtype /* , aTimestamp, aExtraSizeData, aExtraStringData */) {
+    // Sometimes we get a NullHttpChannel, which implements
+    // nsIHttpChannel but not nsIChannel.
+    if (!(nativeChannel instanceof Ci.nsIChannel)) {
+      return;
+    }
     let channel = ChannelWrapper.get(nativeChannel);
 
     // StartStopListener has to be activated early in the request to catch
     // SSL connection issues which do not get reported via nsIHttpActivityObserver.
     if (activityType == nsIHttpActivityObserver.ACTIVITY_TYPE_HTTP_TRANSACTION &&
         activitySubtype == nsIHttpActivityObserver.ACTIVITY_SUBTYPE_REQUEST_HEADER) {
       this.attachStartStopListener(channel);
     }