diff --git a/optika/materials/_tests/test_multilayers.py b/optika/materials/_tests/test_multilayers.py
index 7b31430..e4855e8 100644
--- a/optika/materials/_tests/test_multilayers.py
+++ b/optika/materials/_tests/test_multilayers.py
@@ -49,7 +49,7 @@
 @pytest.mark.parametrize(
     argnames="normal",
     argvalues=[
-        na.Cartesian3dVectorArray(0, 0, -1),
+        None,
     ],
 )
 @pytest.mark.parametrize(