innodb_fast_alter_column doesn't work with indexed columns

mysql> create table t1 (a varchar(20)) engine=innodb;
Query OK, 0 rows affected (0.02 sec)

mysql> alter table t1 modify a varchar(80);
Query OK, 0 rows affected (3.29 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> alter table t1 add key (a);
Query OK, 0 rows affected (1.55 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> alter table t1 modify a varchar(81);
ERROR 1280 (42000): Incorrect index name 'a'

Additionally, that error occurs regardless of whether innodb_fast_alter_column is on or off.