diff --git a/tests/utils/__init__.py b/tests/utils/__init__.py
index a00d586fe1..9e38e87bc2 100644
--- a/tests/utils/__init__.py
+++ b/tests/utils/__init__.py
@@ -474,7 +474,7 @@ def dump_stderr():
 
 
 @contextlib.contextmanager
-def cd_clean_tmp_dir(assert_filenames_after=[]):
+def cd_clean_tmp_dir(assert_filenames_after=None):
     """Run commands inside a clean temporary directory, and verify created file names."""
     orig_cwd = os.getcwd()
     with tempfile.TemporaryDirectory() as tmp_dirname:
@@ -483,6 +483,7 @@ def cd_clean_tmp_dir(assert_filenames_after=[]):
         try:
             yield tmp_dirname
             actual_filenames = os.listdir('.')
-            assert actual_filenames == assert_filenames_after, (actual_filenames, assert_filenames_after)
+            if assert_filenames_after is not None:
+                assert actual_filenames == assert_filenames_after, (actual_filenames, assert_filenames_after)
         finally:
             os.chdir(orig_cwd)