diff --git a/catfish/CatfishWindow.py b/catfish/CatfishWindow.py
index 7acdb48d..3d328531 100644
--- a/catfish/CatfishWindow.py
+++ b/catfish/CatfishWindow.py
@@ -318,8 +318,9 @@ class CatfishWindow(Window):
 
     def get_display_size(self):
         s = self.get_screen()
-        m = s.get_monitor_at_window(s.get_active_window())
-        monitor = s.get_monitor_geometry(m)
+        d = self.get_display()
+        m = d.get_monitor_at_window(s.get_root_window())
+        monitor = m.get_geometry()
         return (monitor.width, monitor.height)
 
     def on_calendar_day_changed(self, widget):