diff --git a/tests/__init__.py b/tests/__init__.py
index 3306f05..7e1de65 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -257,9 +257,9 @@ class GrayTest:
             plt.show()
 
         for ref, cand in zip(ref, cand):
-            with self.subTest(ray=(int(ref['j']), int(ref['k']))):
-                self.assertAlmostEqual(ref['x'], cand['x'], 1)
-                self.assertAlmostEqual(ref['y'], cand['y'], 1)
+            ray = f"ray ({ref['j']:g},{ref['k']:g}) changed"
+            self.assertAlmostEqual(ref['x'], cand['x'], 1, msg=ray)
+            self.assertAlmostEqual(ref['y'], cand['y'], 1, msg=ray)
 
     def test_error_biased(self):
         '''