diff --git a/test/unit/test_autoflush.py b/test/unit/test_autoflush.py
index 2579055a7..8560f7300 100644
--- a/test/unit/test_autoflush.py
+++ b/test/unit/test_autoflush.py
@@ -21,10 +21,12 @@
 from pyocd.core.exceptions import TransferError
 from pyocd.utility.autoflush import Autoflush
 
+
 @pytest.fixture(scope='function')
 def mock_obj():
     return mock.Mock()
 
+
 class TestAutoflush:
     def test_flushed(self, mock_obj):
         with Autoflush(mock_obj):
@@ -35,5 +37,4 @@ def test_transfer_err_not_flushed(self, mock_obj):
         with pytest.raises(TransferError):
             with Autoflush(mock_obj):
                 raise TransferError("bad joojoo")
-        assert mock_obj.flush.not_called
-
+        mock_obj.flush.assert_not_called()