diff --git a/test/unit/ui_utils_spec.js b/test/unit/ui_utils_spec.js
index 145b30d6a..1bbb175d6 100644
--- a/test/unit/ui_utils_spec.js
+++ b/test/unit/ui_utils_spec.js
@@ -80,6 +80,12 @@ describe('ui_utils', function() {
       expect(getPDFFileNameFromURL('/pdfs/file3.txt', '')).toEqual('');
     });
 
+    it('gets fallback filename when url is not a string', function() {
+      expect(getPDFFileNameFromURL(null)).toEqual('document.pdf');
+
+      expect(getPDFFileNameFromURL(null, 'file.pdf')).toEqual('file.pdf');
+    });
+
     it('gets PDF filename from URL containing leading/trailing whitespace',
         function() {
       // Relative URL
diff --git a/web/ui_utils.js b/web/ui_utils.js
index d7806aa8a..940ca81db 100644
--- a/web/ui_utils.js
+++ b/web/ui_utils.js
@@ -548,6 +548,9 @@ function isDataSchema(url) {
  * @returns {string} Guessed PDF filename.
  */
 function getPDFFileNameFromURL(url, defaultFilename = 'document.pdf') {
+  if (typeof url !== 'string') {
+    return defaultFilename;
+  }
   if (isDataSchema(url)) {
     console.warn('getPDFFileNameFromURL: ' +
                  'ignoring "data:" URL for performance reasons.');