diff --git a/date_range/wizard/date_range_generator.py b/date_range/wizard/date_range_generator.py
index fd89907..a474d6d 100644
--- a/date_range/wizard/date_range_generator.py
+++ b/date_range/wizard/date_range_generator.py
@@ -26,10 +26,10 @@ class DateRangeGenerator(models.TransientModel):
         comodel_name='res.company', string='Company',
         default=_default_company)
     unit_of_time = fields.Selection([
-        (YEARLY, 'years'),
-        (MONTHLY, 'months'),
-        (WEEKLY, 'weeks'),
-        (DAILY, 'days')], required=True)
+        ('YEARLY', 'years'),
+        ('MONTHLY', 'months'),
+        ('WEEKLY', 'weeks'),
+        ('DAILY', 'days')], required=True)
     duration_count = fields.Integer('Duration', required=True)
     count = fields.Integer(
         string="Number of ranges to generate", required=True)