EXPLAIN shows non-sensical value for key_len with type=index

create table ten(a int);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t1 (

  pk varchar(50),

  a varchar(20),

  filler varchar(100),

  index(a),

  primary key(pk)

) engine=innodb;

insert into t1 select a,a,a from test.one_k;

explain select pk from t1 order by a;

id      select_type     table   type    possible_keys   key     key_len ref     rows    Extra

1       SIMPLE  t1      index   NULL    a       25      NULL    1120    Using index