BUG: raise when sort_index with ascending=None

pandas/core/generic.py

@@ -4533,6 +4533,8 @@ def sort_index(
 
         inplace = validate_bool_kwarg(inplace, "inplace")
         axis = self._get_axis_number(axis)
+        ascending = validate_ascending(ascending)
+
         target = self._get_axis(axis)
 
         indexer = get_indexer_indexer(
@@ -11772,3 +11774,25 @@ def _doc_params(cls):
     The required number of valid values to perform the operation. If fewer than
     ``min_count`` non-NA values are present the result will be NA.
 """
+
+
+def validate_ascending(
+    ascending: Union[bool, Sequence[bool]],
+) -> Union[bool, Sequence[bool]]:
+    """Validate ``ascending`` kwargs for ``sort_index`` method."""
+    if not isinstance(ascending, (list, tuple)):
+        _check_ascending_element(ascending)
+        return ascending
+
+    for item in ascending:
+        _check_ascending_element(item)
+    return ascending
+
+
+def _check_ascending_element(value):
+    """Ensure that each item in ``ascending`` kwarg is either bool or int."""
+    if (
+        value is None
+        or not isinstance(value, (bool, int))
+    ):
+        raise ValueError("ascending must be either a bool or a sequence of bools")

pandas/tests/frame/methods/test_sort_index.py

@@ -761,6 +761,22 @@ def test_sort_index_with_categories(self, categories):
         )
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "ascending",
+        [
+            None,
+            (True, None),
+            (False, 'True'),
+        ],
+    )
+    def test_sort_index_ascending_bad_value_raises(self, ascending):
+        df = DataFrame(np.arange(64))
+        length = len(df.index)
+        df.index = [(i - length / 2) % length for i in range(length)]
+        match = "ascending must be either a bool or a sequence of bools"
+        with pytest.raises(ValueError, match=match):
+            df.sort_index(axis=0, ascending=ascending, na_position="first")
+
 
 class TestDataFrameSortIndexKey:
     def test_sort_multi_index_key(self):

pandas/tests/series/methods/test_sort_index.py

@@ -198,6 +198,20 @@ def test_sort_index_ascending_list(self):
         expected = ser.iloc[[0, 4, 1, 5, 2, 6, 3, 7]]
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "ascending",
+        [
+            None,
+            (True, None),
+            (False, 'True'),
+        ],
+    )
+    def test_sort_index_ascending_bad_value_raises(self, ascending):
+        ser = Series(range(10), index=[0, 3, 2, 1, 4, 5, 7, 6, 8, 9])
+        match = "ascending must be either a bool or a sequence of bools"
+        with pytest.raises(ValueError, match=match):
+            ser.sort_index(ascending=ascending)
+
 
 class TestSeriesSortIndexKey:
     def test_sort_index_multiindex_key(self):