diff --git a/comet/account/models.py b/comet/account/models.py
index d042805..3ef2878 100644
--- a/comet/account/models.py
+++ b/comet/account/models.py
@@ -19,7 +19,7 @@ class Account(models.Model):
         """Adjust the ballance according to the sales,
         without saving the model."""
         self.balance = Sale.objects.filter(account=self)\
-            .aggregate(models.Sum('price'))['price__sum']
+            .aggregate(models.Sum('price'))['price__sum'] or 0
 
 
 class Sale(models.Model):