diff --git a/prosperon/prosperon.cm b/prosperon/prosperon.cm
index a4f86d93..a7049c64 100644
--- a/prosperon/prosperon.cm
+++ b/prosperon/prosperon.cm
@@ -424,10 +424,9 @@ function poll_input() {
           ev.key = input.keyname(ev.key)
       }
       
-      if (ev.type.startsWith('mouse_'))
+      if (ev.type.startsWith('mouse_') && ev.pos && ev.pos.y)
         ev.pos.y = -ev.pos.y + logical.height
     }
-        
     input_cb(evs)
   })
   $_.delay(poll_input, input_rate)