diff --git a/apply-css/background.js b/apply-css/background.js
index 039a6974..8846b0b1 100644
--- a/apply-css/background.js
+++ b/apply-css/background.js
@@ -27,6 +27,7 @@ function toggleCSS(tab) {
 
 /*
 Returns true only if the URL's protocol is in APPLICABLE_PROTOCOLS.
+Argument url must be a valid URL string.
 */
 function protocolIsApplicable(url) {
   const protocol = (new URL(url)).protocol;