diff --git a/src/calibre/library/custom_columns.py b/src/calibre/library/custom_columns.py
index d925f7c91d..d905f6d01a 100644
--- a/src/calibre/library/custom_columns.py
+++ b/src/calibre/library/custom_columns.py
@@ -151,6 +151,8 @@ class CustomColumns(object):
             return v
 
         def adapt_number(x, d):
+            if x is None:
+                return None
             if isinstance(x, (str, unicode, bytes)):
                 if x.lower() == 'none':
                     return None