diff --git a/trytond/model/modelsql.py b/trytond/model/modelsql.py
index ac5a5604bb..0ace0b7fdb 100644
--- a/trytond/model/modelsql.py
+++ b/trytond/model/modelsql.py
@@ -539,6 +539,8 @@ class ModelSQL(ModelStorage):
                         '%s,%s' % (cls.__name__, id_))
                 except KeyError:
                     continue
+                if timestamp is None:
+                    continue
                 sql_type = fields.Char('timestamp').sql_type().base
                 where.append((table.id == id_)
                     & (Extract('EPOCH',
diff --git a/trytond/tests/test_modelsql.py b/trytond/tests/test_modelsql.py
index 5c8b079eac..2e73b85f72 100644
--- a/trytond/tests/test_modelsql.py
+++ b/trytond/tests/test_modelsql.py
@@ -305,6 +305,10 @@ class ModelSQLTestCase(unittest.TestCase):
         self.assertRaises(ConcurrencyException,
             ModelsqlTimestamp.delete, [record])
 
+        transaction.timestamp[str(record)] = None
+        ModelsqlTimestamp.write([record], {})
+        transaction.commit()
+
         transaction.timestamp.pop(str(record), None)
         ModelsqlTimestamp.write([record], {})
         transaction.commit()