diff --git a/qutebrowser/app.py b/qutebrowser/app.py
index 947792b5f..459c2f76b 100644
--- a/qutebrowser/app.py
+++ b/qutebrowser/app.py
@@ -340,6 +340,7 @@ def _save_version():
     state_config['general']['version'] = qutebrowser.__version__
 
 
+@pyqtSlot('QWidget*', 'QWidget*')
 def on_focus_changed(_old, new):
     """Register currently focused main window in the object registry."""
     if new is None: