diff --git a/src/js/dynamic-net-filtering.js b/src/js/dynamic-net-filtering.js
index 312cb322c..4db9e1d92 100644
--- a/src/js/dynamic-net-filtering.js
+++ b/src/js/dynamic-net-filtering.js
@@ -26,6 +26,8 @@
 
 µBlock.Firewall = (function() {
 
+'use strict';
+
 /******************************************************************************/
 
 var magicId = 'chmdgxwtetgu';