diff --git a/lib/filter.js b/lib/filter.js
index e7e4ecf078e..cb0f9ba8fad 100644
--- a/lib/filter.js
+++ b/lib/filter.js
@@ -6,4 +6,6 @@
 * LICENSE file in the root directory of this source tree.
 */
 
+'use strict';
+
 module.exports = require('../src/transforms/filter');