diff --git a/src/angular-simple-mask.js b/src/angular-simple-mask.js
index 6d9d495..9541120 100644
--- a/src/angular-simple-mask.js
+++ b/src/angular-simple-mask.js
@@ -1,3 +1,5 @@
+'use strict';
+
 angular.module('angularMask', [])
   .directive('angularMask', function() {
   return {