diff --git a/instances/tests/test_live_dataloss.py b/instances/tests/test_live_dataloss.py
index 0079f278..eb8700e2 100644
--- a/instances/tests/test_live_dataloss.py
+++ b/instances/tests/test_live_dataloss.py
@@ -529,7 +529,9 @@ class LiveDataLossTestCase(TestCase):
         before = (self.disk_sources(dom), self.disk_sources(dom, libvirt.VIR_DOMAIN_XML_INACTIVE))
         response = self.post("add_new_vol", inst, {"storage": livetest.POOL, "name": P + "sata-new", "format": "qcow2",
                                                    "size": "1", "bus": "sata", "cache": "default"})
-        self.assertEqual(response.status_code, 500)
+        # libvirt refuses it: the form goes back to the VM page with the reason
+        self.assertRedirects(response, reverse("instances:instance", args=[inst.id]), fetch_redirect_response=False)
+        self.assertTrue(any("libvirt Error" in str(m) for m in get_messages(response.wsgi_request)))
         self.assertEqual((self.disk_sources(dom), self.disk_sources(dom, libvirt.VIR_DOMAIN_XML_INACTIVE)), before)
         # the new volume stays in the pool: it is never deleted unasked
         self.assertIn(P + "sata-new.qcow2", self.conn.storagePoolLookupByName(livetest.POOL).listVolumes())
diff --git a/vrtManager/tests/test_migration.py b/vrtManager/tests/test_migration.py
index 61bb949e..444053dc 100644
--- a/vrtManager/tests/test_migration.py
+++ b/vrtManager/tests/test_migration.py
@@ -472,8 +472,12 @@ class NvramCopyTestCase(unittest.TestCase):
         self.shared.start()
 
     def test_the_copy_is_deleted_when_the_definition_fails(self):
-        self.host.defineXML = mock.MagicMock(side_effect=libvirt.libvirtError("define failed"))
-        with self.assertRaisesRegex(libvirt.libvirtError, "define failed"):
+        # the raised error is this one: its message can be another libvirt
+        # error the thread recorded (a closed connection of an earlier live test)
+        error = libvirt.libvirtError("define failed")
+        self.host.defineXML = mock.MagicMock(side_effect=error)
+        with self.assertRaises(libvirt.libvirtError) as raised:
             self.move()
+        self.assertIs(raised.exception, error)
         self.dest_pool.storageVolLookupByName.assert_called_once_with("vm_VARS.fd")
         self.dest_pool.storageVolLookupByName.return_value.delete.assert_called_once_with(0)