diff --git a/dbus/dbus-mainloop.c b/dbus/dbus-mainloop.c
index 03e9d1c1..a25b31d6 100644
--- a/dbus/dbus-mainloop.c
+++ b/dbus/dbus-mainloop.c
@@ -290,7 +290,11 @@ _dbus_loop_remove_watch (DBusLoop          *loop,
                          void             *data)
 {
   DBusList *link;
-  
+
+  /* fd.o #33336: we want people to remove their watches before invalidating
+   * them */
+  _dbus_assert (dbus_watch_get_socket (watch) != -1);
+
   link = _dbus_list_get_first_link (&loop->callbacks);
   while (link != NULL)
     {
diff --git a/dbus/dbus-watch.c b/dbus/dbus-watch.c
index 8d759e5e..f4a5820e 100644
--- a/dbus/dbus-watch.c
+++ b/dbus/dbus-watch.c
@@ -129,6 +129,9 @@ _dbus_watch_unref (DBusWatch *watch)
   watch->refcount -= 1;
   if (watch->refcount == 0)
     {
+      if (watch->fd != -1)
+        _dbus_warn ("this watch should have been invalidated");
+
       dbus_watch_set_data (watch, NULL, NULL); /* call free_data_function */
 
       if (watch->free_handler_data_function)