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