pg16: XX000: could not find pathkey item to sort

This fails since 1349d2790b

commit 1349d2790bf48a4de072931c722f39337e72055e
Author: David Rowley <drowley@postgresql.org>
Date:   Tue Aug 2 23:11:45 2022 +1200

    Improve performance of ORDER BY / DISTINCT aggregates

ts=# CREATE TABLE t (a int, b text) PARTITION BY RANGE (a);
ts=# CREATE TABLE td PARTITION OF t DEFAULT;
ts=# INSERT INTO t SELECT 1 AS a, '' AS b;
ts=# SET enable_partitionwise_aggregate=on;
ts=# explain SELECT a, COUNT(DISTINCT b) FROM t GROUP BY a;
ERROR:  XX000: could not find pathkey item to sort
LOCATION:  prepare_sort_from_pathkeys, createplan.c:6235

-- 
Justin