diff --git a/setup.py b/setup.py
index fab20a58ee..936c043164 100644
--- a/setup.py
+++ b/setup.py
@@ -41,7 +41,7 @@ if minor_version % 2:
         'hg+http://hg.tryton.org/modules/%s#egg=%s-%s' % (
             name[8:], name, version))
 
-requires = ['python-sql']
+requires = ['python-sql>0.4']
 for dep in info.get('depends', []):
     if not re.match(r'(ir|res|webdav)(\W|$)', dep):
         requires.append(get_require_version('trytond_%s' % dep))
diff --git a/work.py b/work.py
index 7f444018a3..1342eb4455 100644
--- a/work.py
+++ b/work.py
@@ -2,6 +2,8 @@
 #this repository contains the full copyright notices and license terms.
 from itertools import groupby
 from decimal import Decimal
+
+from sql import Null
 from sql.aggregate import Sum
 
 from trytond.model import ModelView, fields
@@ -190,9 +192,9 @@ class Work:
         for sub_ids in grouped_slice(ids):
             red_sql = reduce_ids(line.work, sub_ids)
             if invoiced:
-                where = line.invoice_line != None
+                where = line.invoice_line != Null
             else:
-                where = line.invoice_line == None
+                where = line.invoice_line == Null
             cursor.execute(*line.select(line.work, Sum(line.hours),
                     where=red_sql & where,
                     group_by=line.work))