diff --git a/src/calibre/gui2/dbus_export/tray.py b/src/calibre/gui2/dbus_export/tray.py
index 85e02a0a85..07483de1e6 100644
--- a/src/calibre/gui2/dbus_export/tray.py
+++ b/src/calibre/gui2/dbus_export/tray.py
@@ -192,7 +192,7 @@ class StatusNotifierItemAPI(Object):
     def Menu(self):
         return dbus.ObjectPath(self.dbus_menu.object_path)
 
-    @dbus_property(IFACE, signature='i')
+    @dbus_property(IFACE, signature='u')
     def WindowId(self):
         return 0