unable to drop index because it does not exists

Hi,
running 11.5 I've got a partitioned table where I want to destroy an
index (on a column that has nothing to do with the partition):


respidb=# \d+ respi.root

    Column    |            Type             |
--------------+-----------------------------+-
 sen_id       | integer                     |
 mis_flusso   | integer                     |
 mis_tasso    | integer                     |
 mis_velocita | integer                     |
 mis_ora      | timestamp without time zone |
 pk           | bigint                      |
 ts           | timestamp without time zone |
Partition key: LIST (date_part('year'::text, mis_ora))
Indexes:
    "idx_root_sensore" btree (sen_id)
    "idx_ts" btree (ts)
Partitions: respi.y2018 FOR VALUES IN ('2018'), PARTITIONED,
            respi.y2019 FOR VALUES IN ('2019'), PARTITIONED,
            respi.y2020 FOR VALUES IN ('2020'), PARTITIONED

respidb=# drop index idx_root_sensore;
ERROR:  index "idx_root_sensore" does not exist

Of course the index exists:

SELECT oid, relname, relkind FROM pg_class WHERE relname = 'idx_root_sensore';
-[ RECORD 1 ]-------------
oid     | 40950
relname | idx_root_sensore
relkind | I


I already did a manual vacuum on the table.
Any idea?

Luca