diff --git a/tests/test_cliutil.py b/tests/test_cliutil.py
index 510d6152c6d8d2f9ca7723a379d580c018774a9f..b0425ed277af21864fb26b3d250ea8da9454680b 100644
--- a/tests/test_cliutil.py
+++ b/tests/test_cliutil.py
@@ -298,3 +298,36 @@ def test_enum_arg_choices_str_defaults(arg_enum):
 def test_enum_arg_choices_str_args(arg_enum):
     sep = '/'
     assert arg_enum.choices_str(sep, '{}') == sep.join(c.value for c in ArgChoices)
+
+@pytest.mark.parametrize('values,sep', testutil.combine_values(
+    [['foo'], ['bar', 'baz'], ['qu', 'quu', 'quux']],
+    [',', ', ', '  ,', ' ,  '],
+))
+def test_extend_action_once(values, sep):
+    action = cliutil.ExtendAction(['-t'], 'result')
+    args = argparse.Namespace()
+    action(None, args, sep.join(values), '-t')
+    assert args.result == values
+
+def test_extend_action_multi():
+    action = cliutil.ExtendAction(['-t'], 'result')
+    args = argparse.Namespace()
+    action(None, args, 'foo,bar', '-t')
+    action(None, args, 'baz, quux', '-t')
+    assert args.result == ['foo', 'bar', 'baz', 'quux']
+
+def test_extend_action_from_default():
+    action = cliutil.ExtendAction(['-t'], 'result')
+    args = argparse.Namespace(result=['foo'])
+    action(None, args, 'bar , baz', '-t')
+    assert args.result == ['foo', 'bar', 'baz']
+
+@pytest.mark.parametrize('pattern,expected', [
+    (',', ['foo', ' bar']),
+    (r'\s+', ['foo,', 'bar']),
+])
+def test_extend_action_custom_pattern(pattern, expected):
+    action = cliutil.ExtendAction(['-t'], 'result', const=pattern)
+    args = argparse.Namespace()
+    action(None, args, 'foo, bar', '-t')
+    assert args.result == expected