diff --git a/vncviewer/Viewport.cxx b/vncviewer/Viewport.cxx
index 81dc7ac..ec48b02 100644
--- a/vncviewer/Viewport.cxx
+++ b/vncviewer/Viewport.cxx
@@ -534,6 +534,8 @@
     keySym = XK_Super_L;
     break;
   case XK_Alt_L:
+    keySym = XK_Mode_switch;
+    break;
   case XK_Alt_R:
     keySym = XK_ISO_Level3_Shift;
     break;