Crash in CREATE..SELECT..UNION with a ENUM column and NULL

CREATE OR REPLACE TABLE t1 (a ENUM('a1'));

CREATE OR REPLACE TABLE t2 AS SELECT (SELECT a FROM t1 UNION SELECT NULL);

#0  0x00007ffff5ddaa28 in raise () from /lib64/libc.so.6

#1  0x00007ffff5ddc62a in abort () from /lib64/libc.so.6

#2  0x00007ffff5dd3227 in __assert_fail_base () from /lib64/libc.so.6

#3  0x00007ffff5dd32d2 in __assert_fail () from /lib64/libc.so.6

#4  0x0000555555d13f72 in Type_handler_enum::make_table_field (

    this=0x5555571cd9c8 <type_handler_enum>, name=0x7ffe70013b90, addr=..., 

    attr=..., table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.cc:1628

#5  0x0000555555d12cba in Type_handler::make_and_init_table_field (

    this=0x5555571cd9c8 <type_handler_enum>, name=0x7ffe70013b90, addr=..., 

    attr=..., table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.cc:1218

#6  0x0000555555a7c89b in Item::tmp_table_field_from_field_type (

    this=0x7ffe70013b40, table=0x7ffff4197260)

    at /home/bar/maria-git/server.10.2-ext/sql/item.h:553

CREATE OR REPLACE TABLE t1 (a ENUM('a1'));

CREATE OR REPLACE TABLE t2 AS SELECT (SELECT a FROM t1);