diff --git a/chapters/ch04.0-logtar-our-logging-library.md b/chapters/ch04.0-logtar-our-logging-library.md
index 258e33d..af225f0 100644
--- a/chapters/ch04.0-logtar-our-logging-library.md
+++ b/chapters/ch04.0-logtar-our-logging-library.md
@@ -340,6 +340,9 @@ module.exports = {
 Let's try to test this.
 
 ```js
+
+'use strict'; 
+
 new Logger("OK"); // throws error
 new Logger(LogLevel.Debug); // works fine
 new Logger(); // works fine