diff --git a/tests/test_utils_concat.py b/tests/test_utils_concat.py
index 1c370729..2344a0e9 100644
--- a/tests/test_utils_concat.py
+++ b/tests/test_utils_concat.py
@@ -552,7 +552,6 @@ def test_concat(objs, overwrite, expected):
         pd.testing.assert_frame_equal(obj, expected)
 
 
-#
 @pytest.mark.parametrize(
     'objs, aggregate_function, expected',
     [
@@ -900,3 +899,64 @@ def test_concat_aggregate_function(objs, aggregate_function, expected):
         pd.testing.assert_series_equal(obj, expected)
     else:
         pd.testing.assert_frame_equal(obj, expected)
+
+
+@pytest.mark.parametrize(
+    'objs, aggregate_function, expected',
+    [
+        # empty
+        (
+            [],
+            None,
+            pd.Series([], pd.Index([]), dtype='object'),
+        ),
+        # identical values
+        (
+            [
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+            ],
+            None,
+            pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+        ),
+        (
+            [
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+            ],
+            np.mean,
+            pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+        ),
+        # different values
+        (
+            [
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+                pd.Series([2, 3], pd.Index(['a', 'b']), dtype='float'),
+            ],
+            None,
+            pd.Series([2, 3], pd.Index(['a', 'b']), dtype='float'),
+        ),
+        (
+            [
+                pd.Series([1, 2], pd.Index(['a', 'b']), dtype='float'),
+                pd.Series([2, 3], pd.Index(['a', 'b']), dtype='float'),
+            ],
+            np.mean,
+            pd.Series([2, 3], pd.Index(['a', 'b']), dtype='float'),
+        ),
+    ]
+)
+def test_concat_overwrite_aggregate_function(
+        objs,
+        aggregate_function,
+        expected,
+):
+    obj = audformat.utils.concat(
+        objs,
+        overwrite=True,
+        aggregate_function=aggregate_function,
+    )
+    if isinstance(obj, pd.Series):
+        pd.testing.assert_series_equal(obj, expected)
+    else:
+        pd.testing.assert_frame_equal(obj, expected)