diff --git a/js/phet-io/bumper-phet-io-types.js b/js/phet-io/bumper-phet-io-types.js
index 7daa6ba..639f8df 100644
--- a/js/phet-io/bumper-phet-io-types.js
+++ b/js/phet-io/bumper-phet-io-types.js
@@ -530,7 +530,7 @@ window.phet.phetio.phetioTypes = assert &&
       "events": [],
       "methodOrder": [],
       "methods": {},
-      "supertype": "Object",
+      "supertype": null,
       "typeName": "ObjectIO"
     },
     "PhetButtonIO": {