diff --git a/pd/nw/pdgui.js b/pd/nw/pdgui.js
index 50ba0650619270d9609f6995c5ae8157105629b2..9b1b36811dc7b3fcb308ce55c9e27a07de6468bc 100644
--- a/pd/nw/pdgui.js
+++ b/pd/nw/pdgui.js
@@ -1979,7 +1979,10 @@ function gui_configure_io(cid, tag, is_iemgui, is_signal, width) {
 
 function gui_highlight_io(cid, tag) {
     var xlet = get_item(cid, tag);
-    xlet.classList.add('xlet_selected');
+    // must check for null (see gui_configure_io)
+    if (xlet !== null) {
+        xlet.classList.add('xlet_selected');
+    }
 }
 
 function gui_message_drawborder(cid,tag,width,height) {