diff --git a/lib/6to5/traversal/scope.js b/lib/6to5/traversal/scope.js
index 5d61ec00ec..e02a4165c0 100644
--- a/lib/6to5/traversal/scope.js
+++ b/lib/6to5/traversal/scope.js
@@ -318,7 +318,7 @@ Scope.prototype.crawl = function () {
 
   if (t.isFunction(block)) {
     for (i = 0; i < block.params.length; i++) {
-      this.register(block.params[i], null, "var");
+      this.register(block.params[i], null, "let");
     }
     this.traverse(block.body, blockVariableVisitor, this);
   }