diff --git a/uncloud_pay/models.py b/uncloud_pay/models.py
index 94961c2..66ad4d2 100644
--- a/uncloud_pay/models.py
+++ b/uncloud_pay/models.py
@@ -430,6 +430,7 @@ class Bill(models.Model):
             on_delete=models.CASCADE)
 
     creation_date = models.DateTimeField(auto_now_add=True)
+
     # FIXME: this is a race condition, if ending_date is evaluated
     # in the next month the bill spawns two months!
     starting_date = models.DateTimeField(default=start_of_this_month)