diff --git a/budget/forms.py b/budget/forms.py
index 5abbd845..d0a60ca7 100644
--- a/budget/forms.py
+++ b/budget/forms.py
@@ -120,6 +120,10 @@ class BillForm(Form):
     def set_default(self):
         self.payed_for.data = self.payed_for.default
 
+    def validate_amount(self, field):
+        if field.data < 0:
+            raise ValidationError(_("Bills can't be negative"))
+
 
 class MemberForm(Form):
 
diff --git a/budget/messages.pot b/budget/messages.pot
index b8de26f6..ef1fe234 100644
--- a/budget/messages.pot
+++ b/budget/messages.pot
@@ -445,3 +445,6 @@ msgstr ""
 
 msgid "each"
 msgstr ""
+
+msgid "Bills can't be negative"
+msgstr ""
diff --git a/budget/tests.py b/budget/tests.py
index 0caf1863..452f71fd 100644
--- a/budget/tests.py
+++ b/budget/tests.py
@@ -283,6 +283,15 @@ class BudgetTestCase(TestCase):
         bill = models.Bill.query.one()
         self.assertEqual(bill.amount, 25)
 
+        self.app.post("/raclette/add", data={
+            'date': '2011-08-10',
+            'what': u'fromage à raclette',
+            'payer': members_ids[0],
+            'payed_for': members_ids,
+            'amount': '-25', # bill with a negative value is not possible
+        })
+        self.assertEqual(1, models.Bill.query.count())
+
         # edit the bill
         resp = self.app.post("/raclette/edit/%s" % bill.id, data={
             'date': '2011-08-10',
diff --git a/budget/translations/fr/LC_MESSAGES/messages.mo b/budget/translations/fr/LC_MESSAGES/messages.mo
index c3b07a91..62efe515 100644
Binary files a/budget/translations/fr/LC_MESSAGES/messages.mo and b/budget/translations/fr/LC_MESSAGES/messages.mo differ
diff --git a/budget/translations/fr/LC_MESSAGES/messages.po b/budget/translations/fr/LC_MESSAGES/messages.po
index 0f021eb0..e6232c0e 100644
--- a/budget/translations/fr/LC_MESSAGES/messages.po
+++ b/budget/translations/fr/LC_MESSAGES/messages.po
@@ -451,3 +451,6 @@ msgstr "et les notifier vous même"
 
 msgid "each"
 msgstr "chacun"
+
+msgid "Bills can't be negative"
+msgstr "Le montant d'une facture ne peut pas être négatif"