diff --git a/wcs_olap/feeder.py b/wcs_olap/feeder.py
index 23181fc..eb87827 100644
--- a/wcs_olap/feeder.py
+++ b/wcs_olap/feeder.py
@@ -388,7 +388,7 @@ CREATE TABLE public.dates AS (SELECT
             labels = None
             if to_insert:
                 self.ex('SELECT MAX(id) FROM {name}', ctx={'name': name})
-                next_id = self.cur.fetchone()[0] + 1
+                next_id = (self.cur.fetchone()[0] or 0) + 1
                 ids = range(next_id, next_id + len(to_insert))
                 labels = zip(ids, to_insert)