diff --git a/lib/utils/templatize.js b/lib/utils/templatize.js
index 7d4ed9dd3f..ff488c15d9 100644
--- a/lib/utils/templatize.js
+++ b/lib/utils/templatize.js
@@ -101,7 +101,10 @@ function upgradeTemplate(template, constructor) {
  * @implements {Polymer_PropertyEffects}
  * @private
  */
-const templateInstanceBase = PropertyEffects(class {});
+const templateInstanceBase = PropertyEffects(
+    // This cast shouldn't be neccessary, but Closure doesn't understand that
+    // "class {}" is a constructor function.
+    /** @type {function(new:Object)} */(class {}));
 
 /**
  * @polymer
@@ -317,7 +320,10 @@ TemplateInstanceBase.prototype.__hostProps;
  * @implements {Polymer_MutableData}
  * @private
  */
-const MutableTemplateInstanceBase = MutableData(TemplateInstanceBase);
+const MutableTemplateInstanceBase = MutableData(
+    // This cast shouldn't be necessary, but Closure doesn't seem to understand
+    // this constructor.
+    /** @type {function(new:TemplateInstanceBase)} */(TemplateInstanceBase));
 
 function findMethodHost(template) {
   // Technically this should be the owner of the outermost template.