diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index 55da05b75b..ef652e2008 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -403,7 +403,7 @@ t.toIdentifier = function (name) {
  * Description
  *
  * @param {Object} node
- * @param {String} key
+ * @param {String=} key
  */
 
 t.ensureBlock = function (node, key) {