diff --git a/index.js b/index.js
index c61e483..4602fe5 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const assert = require('assert').strict
 const Firestore = require('@google-cloud/firestore')
 
diff --git a/test.js b/test.js
index 6fd9548..d4a91e2 100644
--- a/test.js
+++ b/test.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const test = require('ava')
 const testSuite = require('@keyv/test-suite').default
 const Keyv = require('keyv')