Unsuitable error upon an attempt to create MEMORY table with vector key

MariaDB [test]> create table t (v binary(255) not null, vector index(v)) engine=Memory;

ERROR 1939 (HY000): Engine MEMORY failed to discover table `test`.`t` with 'CREATE TABLE i (                     layer tinyint not null,            tref varbinary(8),                vec blob not null,                 neighbors blob not null,           unique (tref),                     key (layer))                     '