diff --git a/catfish/CatfishWindow.py b/catfish/CatfishWindow.py
index 7acdb48d..d077e53d 100644
--- a/catfish/CatfishWindow.py
+++ b/catfish/CatfishWindow.py
@@ -318,7 +318,7 @@ class CatfishWindow(Window):
 
     def get_display_size(self):
         s = self.get_screen()
-        m = s.get_monitor_at_window(s.get_active_window())
+        m = s.get_monitor_at_window(s.get_root_window())
         monitor = s.get_monitor_geometry(m)
         return (monitor.width, monitor.height)