virtual columns don't work with enum or set as data type

Trying to use an ENUM data type for a persistent virtual column results in an error

CREATE TABLE table1 (
a INT not null,
b char(2) not null,
c enum("Y","N") as (case when b = "aa" then "Y" else "N" end) persistent
);

ERROR 1033 (HY000): Incorrect information in file: './database/table1.frm'

Changing the datatype results in a successful table creation.

CREATE TABLE table1 (
a INT not null,
b char(2) not null,
c char(1) as (case when b = "aa" then "Y" else "N" end) persistent
);

Query OK, 0 rows affected (0.16 sec)