diff --git a/tests/unit/backend/dataio/test_data.py b/tests/unit/backend/dataio/test_data.py
index 2745d21d..be814b20 100644
--- a/tests/unit/backend/dataio/test_data.py
+++ b/tests/unit/backend/dataio/test_data.py
@@ -97,6 +97,7 @@ def test_load_data(
     # error_0: incorrect input argument types
     with pytest.raises(ValueError):
         load_data(ct_files=1, ob_files=[], dc_files=[])
+        load_data(ct_dir=1, ob_files=[], dc_files=[])
         load_data(ct_files=[], ob_files=[], dc_files=[], ct_fnmatch=1)
         load_data(ct_files=[], ob_files=[], dc_files=[], ob_fnmatch=1)
         load_data(ct_files=[], ob_files=[], dc_files=[], dc_fnmatch=1)