diff --git a/nhf/test.py b/nhf/test.py
index 37a750d..5695c41 100644
--- a/nhf/test.py
+++ b/nhf/test.py
@@ -7,8 +7,9 @@ class TestJoints(unittest.TestCase):
 
     def test_joint_hirth(self):
         j = nhf.joints.hirth_joint()
-        assert isinstance(j.val().solids(), Cq.Solid),\
-            "Hirth joint must be in one piece"
+        self.assertIsInstance(
+            j.val().solids(), Cq.Solid,
+            msg="Hirth joint must be in one piece")
     def test_joints_hirth_assembly(self):
         nhf.joints.hirth_assembly()
     def test_joints_comma_assembly(self):