diff --git a/lib/polyfills.js b/lib/polyfills.js
index b7fc44e..65d71a2 100644
--- a/lib/polyfills.js
+++ b/lib/polyfills.js
@@ -405,24 +405,22 @@ exports.Element_attachShadow = jss`
       }
       rel.call(target, type, listener, options);
     }
-    function sanityChecks(a, name, cls) {
-        const namDef = a.a.get(name),
-              clsDef = a.g.get(cls);
-        console.dir({
-            _name: name,
-            _cls: cls,
-            regName: namDef,
-            regCls: clsDef,
-            sameConstrName: (namDef?namDef.constructorFunction==cls:undefined),
-            sameConstrClass: (clsDef?clsDef.constructorFunction==cls:undefined),
-        });
-    }
     customElements.define = function (name, cls) {
       asNames.add(name);
-      sanityChecks(customElements.a, name, cls);
       cls.prototype.addEventListener = addCEEventListener;
       cls.prototype.removeEventListener = removeCEEventListener;
-      oldCED.call(customElements, name, cls);
+      try {
+        return oldCED.call(customElements, name, cls);
+      } catch(c) {
+          // rewrite JS exception classes from polyfill to correct DOMException
+          if (c instanceof SyntaxError) {
+            throw new DOMException(c.message, "SyntaxError");
+          } else
+          if (c instanceof Error && c.name==="Error") {
+            throw new DOMException(c.message, "NotSupportedError");
+          } else
+            throw c;
+      }
     };
   }
 }())`;