diff --git a/lib/6to5/transformation/transformers/es6-default-parameters.js b/lib/6to5/transformation/transformers/es6-default-parameters.js
index ba3c0ad631..eb7b6fc1b3 100644
--- a/lib/6to5/transformation/transformers/es6-default-parameters.js
+++ b/lib/6to5/transformation/transformers/es6-default-parameters.js
@@ -31,7 +31,7 @@ exports.Function = function (node, parent, file, scope) {
           throw file.errorWithNode(node, "Temporal dead zone - accessing a variable before it's initialized");
         }
 
-        if (scope.has(node.name)) {
+        if (scope.has(node.name, true)) {
           iife = true;
         }
       };