diff --git a/src/calibre/library/custom_columns.py b/src/calibre/library/custom_columns.py
index 2401ef3026..d65eaa8ecc 100644
--- a/src/calibre/library/custom_columns.py
+++ b/src/calibre/library/custom_columns.py
@@ -532,6 +532,8 @@ class CustomColumns(object):
 
         if data['datatype'] == 'series' and extra is None:
             (val, extra) = self._get_series_values(val)
+            if extra is None:
+                extra = 1.0
 
         books_to_refresh = set([])
         if data['normalized']: