--- a/tests/concave.py	Mon Jan 22 17:05:10 2018 +0200
+++ b/tests/concave.py	Mon Jan 22 17:42:57 2018 +0200
@@ -36,9 +36,9 @@
     },
     'messages': {
         'skew-error': lambda skew_angle:
-            str.format('Skew quadrilateral (plane angle {}°)',
+            str.format('skew quadrilateral (plane angle {}°)',
                 '%.2f' % degrees(skew_angle),
             ),
-        'concave-error': 'Concave quadrilateral',
+        'concave-error': 'concave quadrilateral',
     },
 }