diff --git a/web/source/osk/visualKeyboard.ts b/web/source/osk/visualKeyboard.ts
index 5b30c3641c..99cf62157d 100644
--- a/web/source/osk/visualKeyboard.ts
+++ b/web/source/osk/visualKeyboard.ts
@@ -1921,7 +1921,7 @@ namespace com.keyman.osk {
      **/
     highlightKey(key: KeyElement, on: boolean) {
       // Do not change element class unless a key
-      if(!key || (key.className == '') || (key.className.indexOf('kmw-key-row') >= 0)) return;
+      if(!key || (key.className == '') || (typeof key.className != 'string') || (key.className.indexOf('kmw-key-row') >= 0)) return;
 
       var classes=key.className, cs = ' kmw-key-touched';