diff --git a/lib/iris/tests/unit/util/test_new_axis.py b/lib/iris/tests/unit/util/test_new_axis.py
index 009787f80e..a6374f97ad 100644
--- a/lib/iris/tests/unit/util/test_new_axis.py
+++ b/lib/iris/tests/unit/util/test_new_axis.py
@@ -122,7 +122,9 @@ def test_existing_dim_coord(self, stock_cube):
         stock_cube.add_aux_coord(coord)
 
         new_cube = iris.util.new_axis(stock_cube, coord)
-        with pytest.raises(ValueError, match="is already a dimension coordinate."):
+        with pytest.raises(
+            ValueError, match="is already a dimension coordinate."
+        ):
             iris.util.new_axis(new_cube, coord)
 
     def test_promote_non_scalar(self, stock_cube):