diff --git a/web/app.js b/web/app.js
index ff7e5d912..66bacb9a5 100644
--- a/web/app.js
+++ b/web/app.js
@@ -1403,6 +1403,9 @@ if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
   const HOSTED_VIEWER_ORIGINS = ['null',
     'http://mozilla.github.io', 'https://mozilla.github.io'];
   validateFileURL = function validateFileURL(file) {
+    if (file === undefined) {
+      return;
+    }
     try {
       let viewerOrigin = new URL(window.location.href).origin || 'null';
       if (HOSTED_VIEWER_ORIGINS.indexOf(viewerOrigin) >= 0) {