diff --git a/src/display/display_utils.js b/src/display/display_utils.js
index 2ff4860f2..fe51836ff 100644
--- a/src/display/display_utils.js
+++ b/src/display/display_utils.js
@@ -697,6 +697,9 @@ class StatTimer {
 }
 
 function isValidFetchUrl(url, baseUrl) {
+  if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) {
+    throw new Error("Not implemented: isValidFetchUrl");
+  }
   try {
     const { protocol } = baseUrl ? new URL(url, baseUrl) : new URL(url);
     // The Fetch API only supports the http/https protocols, and not file/ftp.