API: Fix crash in order creation (PRETIXEU-9XS)

src/pretix/api/serializers/order.py

@@ -1439,6 +1439,7 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
                     if not pos.item.tax_rule or pos.item.tax_rule.price_includes_tax:
                         price_after_voucher = max(pos.price, pos.voucher.calculate_price(listed_price))
                     else:
+                        pos._calculate_tax(invoice_address=ia)
                         price_after_voucher = max(pos.price - pos.tax_value, pos.voucher.calculate_price(listed_price))
                 else:
                     price_after_voucher = listed_price
@@ -1466,7 +1467,7 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
             answers_data = pos_data.pop('answers', [])
             use_reusable_medium = pos_data.pop('use_reusable_medium', None)
             pos = pos_data['__instance']
-            pos._calculate_tax()
+            pos._calculate_tax(invoice_address=ia)
 
             if simulate:
                 pos = WrappedModel(pos)

src/pretix/base/models/orders.py

@@ -2582,10 +2582,10 @@ class OrderPosition(AbstractPosition):
             self.item.id, self.variation.id if self.variation else 0, self.order_id
         )
 
-    def _calculate_tax(self, tax_rule=None):
+    def _calculate_tax(self, tax_rule=None, invoice_address=None):
         self.tax_rule = tax_rule or self.item.tax_rule
         try:
-            ia = self.order.invoice_address
+            ia = invoice_address or self.order.invoice_address
         except InvoiceAddress.DoesNotExist:
             ia = None
         if self.tax_rule:

src/tests/api/test_order_create.py

@@ -2190,6 +2190,33 @@ def test_order_create_with_seat_consumed_from_cart(token_client, organizer, even
     assert p.seat == seat
 
 
+@pytest.mark.django_db
+def test_order_create_with_voucher_and_price_set(token_client, organizer, event, item, quota, question):
+    with scopes_disabled():
+        voucher = event.vouchers.create(code="FOOBAR", item=item, price_mode="set", value=0, max_usages=3)
+        item.tax_rule = event.tax_rules.create(rate=19, price_includes_tax=False)
+        item.save()
+    res = copy.deepcopy(ORDER_CREATE_PAYLOAD)
+    res['positions'][0]['item'] = item.pk
+    res['positions'][0]['voucher'] = voucher.code
+    res['positions'][0]['price'] = '0.00'
+    res['positions'][0].pop("tax_rate", None)
+    res['positions'][0].pop("tax_rule", None)
+    res['positions'][0].pop("tax_value", None)
+    res['positions'][0]['answers'][0]['question'] = question.pk
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orders/'.format(
+            organizer.slug, event.slug
+        ), format='json', data=res
+    )
+    assert resp.status_code == 201
+    with scopes_disabled():
+        o = Order.objects.get(code=resp.data['code'])
+        p = o.positions.first()
+    assert p.voucher == voucher
+    assert p.price == Decimal("0.00")
+
+
 @pytest.mark.django_db
 def test_order_create_with_voucher_consumed_from_cart(token_client, organizer, event, item, quota, question):
     with scopes_disabled():