diff --git a/lib/utils.js b/lib/utils.js
index 18637186..0382fe77 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -1,3 +1,5 @@
+'use strict'
+
 function filterPseudoHeaders (headers) {
   const dest = {}
   const headersKeys = Object.keys(headers)