diff --git a/init.lua b/init.lua
index 60bc0f3..5a1de5a 100644
--- a/init.lua
+++ b/init.lua
@@ -40,6 +40,7 @@ client.connect_signal("focus", function(c)
 
     gears.timer.weak_start_new(0.15, function()
         local client_under_mouse = mouse.current_client
+        if client_under_mouse == nil then return false end
         local should_stay = set_contains(stay_classes, client_under_mouse.class)
 
         if should_stay then return false end