diff --git a/src/diffuse/utils.py b/src/diffuse/utils.py
index 27e9a99..92a69ea 100644
--- a/src/diffuse/utils.py
+++ b/src/diffuse/utils.py
@@ -38,7 +38,7 @@ from gi.repository import Gtk  # type: ignore # noqa: E402
 
 # convenience class for displaying a message dialogue
 class MessageDialog(Gtk.MessageDialog):
-    def __init__(self, parent, message_type, s):
+    def __init__(self, parent: Gtk.Widget, message_type: Gtk.MessageType, s: str) -> None:
         if message_type == Gtk.MessageType.ERROR:
             buttons = Gtk.ButtonsType.OK
         else: