ALTER ONLINE TABLE reported as not supported for TokuDB table

Hi,

When I try to run an ALTER ONLINE TABLE my_table ADD KEY my_key (...) on a TokuDB table, it reports
ERROR 1845 (0A000): LOCK=NONE is not supported for this operation. Try LOCK=SHARED.

However, ONLINE is supported, and it's an online operation which is applied when the syntax
CREATE INDEX my_key ON my_table (...) is used if tokudb_create_index_online is ON, as described here : https://www.percona.com/doc/percona-server/5.7/tokudb/using_tokudb.html.

To repeat :

CREATE TABLE test (a INT) ENGINE=TokuDB;

ALTER ONLINE TABLE test ADD KEY my_index (a);

ERROR 1845 (0A000): LOCK=NONE is not supported for this operation. Try LOCK=SHARED.

It would be great to have a consistent behaviour in MariaDB.
(but I agree it should be fixed upstream instead)

Thanks and regards,
Jocelyn Fournier