diff --git a/ihatemoney/forms.py b/ihatemoney/forms.py
index 2a7ba24f..4b000dc0 100644
--- a/ihatemoney/forms.py
+++ b/ihatemoney/forms.py
@@ -382,7 +382,7 @@ class BillForm(FlaskForm):
         self.payed_for.data = self.payed_for.default
 
     def validate_amount(self, field):
-        if field.data == 0:
+        if field.data == "0":
             raise ValidationError(_("Bills can't be null"))
 
 
diff --git a/ihatemoney/tests/api_test.py b/ihatemoney/tests/api_test.py
index f5475b11..f40d8142 100644
--- a/ihatemoney/tests/api_test.py
+++ b/ihatemoney/tests/api_test.py
@@ -910,6 +910,25 @@ class APITestCase(IhatemoneyTestCase):
         self.assertEqual(resp.data.decode("utf-8").count("<td> -- </td>"), 2)
         self.assertNotIn("127.0.0.1", resp.data.decode("utf-8"))
 
+    def test_amount_is_null(self):
+        self.api_create("raclette")
+        # add participants
+        self.api_add_member("raclette", "zorglub")
+
+        # add a bill null amount
+        req = self.client.post(
+            "/api/projects/raclette/bills",
+            data={
+                "date": "2011-08-10",
+                "what": "fromage",
+                "payer": "1",
+                "payed_for": ["1"],
+                "amount": "0",
+            },
+            headers=self.get_auth("raclette"),
+        )
+        self.assertStatus(400, req)
+
     def test_project_creation_with_mixed_case(self):
         self.api_create("Raclette")
         # get information about it
diff --git a/ihatemoney/tests/budget_test.py b/ihatemoney/tests/budget_test.py
index f7781095..17cb63e5 100644
--- a/ihatemoney/tests/budget_test.py
+++ b/ihatemoney/tests/budget_test.py
@@ -1531,6 +1531,30 @@ class BudgetTestCase(IhatemoneyTestCase):
         ]
         assert no_currency_bills == [(5.0, 5.0), (10.0, 10.0)]
 
+    def test_amount_is_null(self):
+        self.post_project("raclette")
+
+        # add participants
+        self.client.post("/raclette/members/add", data={"name": "zorglub"})
+
+        # null amount
+        resp = self.client.post(
+            "/raclette/add",
+            data={
+                "date": "2016-12-31",
+                "what": "fromage à raclette",
+                "payer": 1,
+                "payed_for": [1],
+                "amount": "0",
+                "original_currency": "EUR",
+            },
+        )
+        assert '<p class="alert alert-danger">' in resp.data.decode("utf-8")
+
+        resp = self.client.get("/raclette/")
+        # No bills, the previous one was not added
+        assert "No bills" in resp.data.decode("utf-8")
+
     def test_decimals_on_weighted_members_list(self):
 
         self.post_project("raclette")