diff --git a/apply-css/background.js b/apply-css/background.js
index 3c0b1aad..8846b0b1 100644
--- a/apply-css/background.js
+++ b/apply-css/background.js
@@ -27,11 +27,11 @@ 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) {
-  var anchor =  document.createElement('a');
-  anchor.href = url;
-  return APPLICABLE_PROTOCOLS.includes(anchor.protocol);
+  const protocol = (new URL(url)).protocol;
+  return APPLICABLE_PROTOCOLS.includes(protocol);
 }
 
 /*