diff --git a/lib/6to5/helpers/clone.js b/lib/6to5/helpers/clone.js
index a00125e8c7..ab0cdf09cf 100644
--- a/lib/6to5/helpers/clone.js
+++ b/lib/6to5/helpers/clone.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = function cloneDeep(obj) {
   var obj2 = {};
   if (!obj) return obj2;
diff --git a/lib/6to5/transformation/transformers/es6/block-scoping-tdz.js b/lib/6to5/transformation/transformers/es6/block-scoping-tdz.js
index b97f49ebb0..bdb36fa30b 100644
--- a/lib/6to5/transformation/transformers/es6/block-scoping-tdz.js
+++ b/lib/6to5/transformation/transformers/es6/block-scoping-tdz.js
@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../../traverse");
 var t        = require("../../../types");
 
diff --git a/lib/6to5/transformation/transformers/validation/undeclared-variable-check.js b/lib/6to5/transformation/transformers/validation/undeclared-variable-check.js
index 4655979211..338da22569 100644
--- a/lib/6to5/transformation/transformers/validation/undeclared-variable-check.js
+++ b/lib/6to5/transformation/transformers/validation/undeclared-variable-check.js
@@ -1,3 +1,5 @@
+"use strict";
+
 exports.optional = true;
 
 exports.Identifier = function (node, parent, scope, context, file) {
diff --git a/lib/6to5/types/node.js b/lib/6to5/types/node.js
index 0bea4da80e..19d60d3130 100644
--- a/lib/6to5/types/node.js
+++ b/lib/6to5/types/node.js
@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Node;
 
 var acorn = require("acorn-6to5");