diff --git a/src/Fl_cocoa.mm b/src/Fl_cocoa.mm
index 8ccda6dbb..7df94b879 100644
--- a/src/Fl_cocoa.mm
+++ b/src/Fl_cocoa.mm
@@ -2445,6 +2445,9 @@ static void cocoaKeyboardHandler(NSEvent *theEvent)
 }
 - (void)keyDown:(NSEvent *)theEvent {
   //NSLog(@"keyDown:%@",[theEvent characters]);
+  if (![[theEvent window] isKindOfClass:[FLWindow class]]) { // occurs with #1431
+    return;
+  }
   fl_lock_function();
   Fl_Window *window = [(FLWindow*)[theEvent window] getFl_Window];
   Fl::first_window(window);