From b1326a0ad8a164242ea768675df5af8dd2567653 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Tue, 12 Jul 2022 20:10:44 +0530
Subject: [PATCH] Fix get next series number failing for custom series columns
 with None series index values

---
 src/calibre/db/cache.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 6e80ce725b..cb12905772 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -1831,7 +1831,7 @@ class Cache:
         index_map = {book_id:self._fast_field_for(idf, book_id, default_value=1.0) for book_id in books}
         if current_indices:
             return index_map
-        series_indices = sorted(itervalues(index_map))
+        series_indices = sorted(index_map.values(), key=lambda s: s or 0)
         return _get_next_series_num_for_list(tuple(series_indices), unwrap=False)
 
     @read_api