BUG: Bug in loc raised ValueError when setting value via boolean list

doc/source/whatsnew/v1.2.0.rst

@@ -470,6 +470,7 @@ Indexing
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
 - Bug in :meth:`Series.loc` and :meth:`DataFrame.loc` raises when numeric label was given for object :class:`Index` although label was in :class:`Index` (:issue:`26491`)
 - Bug in :meth:`DataFrame.loc` returned requested key plus missing values when ``loc`` was applied to single level from :class:`MultiIndex` (:issue:`27104`)
+- Bug in :meth:`Series.loc` raised an Error when input was filtered with a boolean list and values to set were a list with lower dimension (:issue:`20438`)
 
 Missing
 ^^^^^^^

pandas/core/indexing.py

@@ -602,7 +602,7 @@ def _get_setitem_indexer(self, key):
         """
         Convert a potentially-label-based key into a positional indexer.
         """
-        if self.name == "loc":
+        if self.name == "loc" or com.is_bool_indexer(key):
             self._ensure_listlike_indexer(key)
 
         if self.axis is not None:

pandas/tests/indexing/test_loc.py

@@ -1644,6 +1644,18 @@ def test_loc_setitem_mask_td64_series_value(self):
         assert expected == result
         tm.assert_frame_equal(df, df_copy)
 
+    def test_loc_setitem_boolean_list(self):
+        # GH#20438 testing specifically list key, not arraylike
+        ser = Series([0, 1, 2])
+        ser.loc[[True, False, True]] = [5, 10]
+        expected = Series([5, 1, 10])
+        tm.assert_series_equal(ser, expected)
+
+        df = DataFrame({"a": [0, 1, 2]})
+        df.loc[[True, False, True]] = [[5], [10]]
+        expected = DataFrame({"a": [5, 1, 10]})
+        tm.assert_frame_equal(df, expected)
+
 
 def test_series_loc_getitem_label_list_missing_values():
     # gh-11428