From 8c02414d845fdef3d0ec4229a0161d7929019eb4 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Fri, 6 Dec 2024 15:55:12 +0530
Subject: [PATCH] Fix #2091151 [MDE: UnboundLocalError in Invalid ISBN
 dialog](https://bugs.launchpad.net/calibre/+bug/2091151)

---
 src/calibre/gui2/metadata/basic_widgets.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/calibre/gui2/metadata/basic_widgets.py b/src/calibre/gui2/metadata/basic_widgets.py
index fe90899983..9037ea946f 100644
--- a/src/calibre/gui2/metadata/basic_widgets.py
+++ b/src/calibre/gui2/metadata/basic_widgets.py
@@ -1854,7 +1854,7 @@ class ISBNDialog(QDialog):  # {{{
         isbn = str(txt)
         ok = None
         if not isbn:
-            pass
+            extra = ''
         elif check_isbn(isbn) is not None:
             extra = _('This ISBN is valid')
             ok = True