diff --git a/externs/closure-types.js b/externs/closure-types.js
index 62e86e785d..48e3167272 100644
--- a/externs/closure-types.js
+++ b/externs/closure-types.js
@@ -1299,6 +1299,10 @@ Polymer_LegacyElementMixin.prototype._error = function(args){};
 */
 Polymer_LegacyElementMixin.prototype._logf = function(methodName, args){};
 /**
+* @return {undefined}
+*/
+Polymer_LegacyElementMixin._finalizeClass = function(){};
+/**
 * @interface
 */
 function Polymer_MutableData(){}