diff --git a/lib/marked.js b/lib/marked.js
index 3eb1f9dfca..045d17afe3 100644
--- a/lib/marked.js
+++ b/lib/marked.js
@@ -5,6 +5,7 @@
  */
 
 ;(function() {
+'use strict';
 
 /**
  * Block-Level Grammar