diff --git a/testapp/testapp/testmain/tests/test_webservices.py b/testapp/testapp/testmain/tests/test_webservices.py
index 218d7eeb..ddf5df0f 100644
--- a/testapp/testapp/testmain/tests/test_webservices.py
+++ b/testapp/testapp/testmain/tests/test_webservices.py
@@ -323,5 +323,20 @@ def test_validation_with_observations(self):
         self.assertEqual(models.Observation.objects.count(), 1)
         self.assertEqual(receipt.validation.observations.count(), 1)
 
+    def test_credit_note(self):
+        """Test validating valid a credit note."""
+        # Create an invoice (code=6) and validate it...
+        invoice = self._good_receipt()
 
-# TODO: Test receipts with related_receipts
+        errs = models.Receipt.objects.filter(pk=invoice.pk).validate()
+        self.assertEqual(len(errs), 0)
+        self.assertEqual(models.ReceiptValidation.objects.count(), 1)
+
+        # Now create a credit note (code=8) and validate it...
+        credit = self._good_receipt()
+        credit.receipt_type = factories.ReceiptTypeFactory(code=8)
+        credit.related_receipts.set([invoice])
+
+        errs = models.Receipt.objects.filter(pk=credit.pk).validate()
+        self.assertEqual(len(errs), 0)
+        self.assertEqual(models.ReceiptValidation.objects.count(), 2)
diff --git a/tox.ini b/tox.ini
index 7a08033d..12fcb065 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,7 @@ envlist =
 skip_install = True
 deps = pipenv
 commands =
-  pipenv install --dev --skip-lock
+  pipenv --bare install --dev --skip-lock
   postgres: pip install -e .[postgres]
   mysql: pip install -e .[mysql]
   django21: pip install "Django>=2.1,<2.2"
@@ -23,7 +23,7 @@ passenv =
 [testenv:live]
 deps = pipenv
 commands =
-  pipenv install --dev
+  pipenv --bare install --dev
   coverage run testapp/manage.py test testapp
   coverage report