diff --git a/lisp/mwheel.el b/lisp/mwheel.el
index de47081b84f..0194160bcf4 100644
--- a/lisp/mwheel.el
+++ b/lisp/mwheel.el
@@ -58,7 +58,7 @@
                         'mouse-wheel-down-event)
 (defcustom mouse-wheel-down-event
   ;; In the latest versions of XEmacs, we could just use mouse-%s as well.
-  (if (memq system-type '(windows-nt macos darwin))
+  (if (memq window-system '(w32 mac))
       'wheel-up
     (intern (format (if (featurep 'xemacs) "button%s" "mouse-%s")
 		    mouse-wheel-down-button)))
@@ -72,7 +72,7 @@
                         'mouse-wheel-up-event)
 (defcustom mouse-wheel-up-event
   ;; In the latest versions of XEmacs, we could just use mouse-%s as well.
-  (if (memq system-type '(windows-nt macos darwin))
+  (if (memq window-system '(w32 mac))
       'wheel-down
     (intern (format (if (featurep 'xemacs) "button%s" "mouse-%s")
 		    mouse-wheel-up-button)))