diff --git a/lib/csp.js b/lib/csp.js
index 9630219..6224b08 100644
--- a/lib/csp.js
+++ b/lib/csp.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { createHash } = require('crypto')
 
 const createCspHeader = (options, {