diff --git a/lib/legacy/legacy-data-mixin.html b/lib/legacy/legacy-data-mixin.html
index b267490fee..af367a3e3b 100644
--- a/lib/legacy/legacy-data-mixin.html
+++ b/lib/legacy/legacy-data-mixin.html
@@ -8,14 +8,19 @@
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
 
-<link rel="import" href="../class.html">
+<link rel="import" href="class.html">
 <link rel="import" href="../utils/mixin.html">
 
 <script>
 (function() {
   'use strict';
 
-  const UndefinedArgumentError = class extends Error {};
+  const UndefinedArgumentError = class extends Error {
+    constructor(message) {
+      super(message);
+      this.name = this.constructor.name;
+    }
+  };
 
   /**
    * Mixin to selectively add back Polymer 1.x's `undefined` rules
@@ -96,7 +101,11 @@
   });
 
   const Class = Polymer.Class;
-  Polymer.Class = info => Class(info, Polymer.LegacyDataMixin);
+  Polymer.Class = (info, mixin) => Class(info, 
+    superClass => mixin ? 
+      mixin(Polymer.LegacyDataMixin(superClass)) : 
+      Polymer.LegacyDataMixin(superClass)
+  );
 
   console.info('LegacyDataMixin will be applied to all legacy elements.\n' +
                'Set `_legacyUndefinedCheck: true` to enable.');
diff --git a/types/lib/legacy/legacy-data-mixin.d.ts b/types/lib/legacy/legacy-data-mixin.d.ts
index 372b9b0420..91d33e9cde 100644
--- a/types/lib/legacy/legacy-data-mixin.d.ts
+++ b/types/lib/legacy/legacy-data-mixin.d.ts
@@ -8,6 +8,7 @@
  *   lib/legacy/legacy-data-mixin.html
  */
 
+/// <reference path="class.d.ts" />
 /// <reference path="../utils/mixin.d.ts" />
 
 declare class UndefinedArgumentError {