diff --git a/src/background.js b/src/background.js
index 688aaa1..d7aae23 100644
--- a/src/background.js
+++ b/src/background.js
@@ -1,3 +1,5 @@
+// IMPORTANT: keep this file in sync with sw.js
+
 // When the extension gets updated, set new options to their default value.
 chrome.runtime.onInstalled.addListener(function(details) {
   if (details.reason == 'install' || details.reason == 'update') {
diff --git a/src/sw.js b/src/sw.js
index c90c5a6..c214068 100644
--- a/src/sw.js
+++ b/src/sw.js
@@ -1,3 +1,5 @@
+// IMPORTANT: keep this file in sync with background.js
+
 importScripts('common/common.js')
 
 // When the extension gets updated, set new options to their default value.