Assertion `tables_opened == 1' failed in mysql_alter_table upon adding FK to a MERGE table

CREATE TABLE t1 (a INT, b INT, PRIMARY KEY (a)) ENGINE=MyISAM;

CREATE TABLE t2 (a INT, b INT, PRIMARY KEY (a)) ENGINE=MERGE UNION=(t1);

ALTER TABLE t2 ADD FOREIGN KEY (a) REFERENCES t2 (b), ALGORITHM=COPY;

# Cleanup

DROP TABLE t2, t1;

mysqld: /data/src/10.1/sql/sql_table.cc:9185: bool mysql_alter_table(THD*, char*, char*, HA_CREATE_INFO*, TABLE_LIST*, Alter_info*, uint, ORDER*, bool): Assertion `tables_opened == 1' failed.

190201  0:38:51 [ERROR] mysqld got signal 6 ;

#7  0x00007f6740b5fee2 in __assert_fail () from /lib/x86_64-linux-gnu/libc.so.6

#8  0x000055e53a404cc0 in mysql_alter_table (thd=0x7f6720000b00, new_db=0x7f67200051c0, new_name=0x7f6720005590, create_info=0x7f6736cceec0, table_list=0x7f67200152b0, alter_info=0x7f6736ccee00, order_num=0, order=0x0, ignore=false) at /data/src/10.4/sql/sql_table.cc:9790

#9  0x000055e53a48fcc8 in Sql_cmd_alter_table::execute (this=0x7f6720015b00, thd=0x7f6720000b00) at /data/src/10.4/sql/sql_alter.cc:499

#10 0x000055e53a32b51e in mysql_execute_command (thd=0x7f6720000b00) at /data/src/10.4/sql/sql_parse.cc:6306

#11 0x000055e53a330658 in mysql_parse (thd=0x7f6720000b00, rawbuf=0x7f6720015178 "ALTER TABLE t2 ADD FOREIGN KEY (a) REFERENCES t2 (b), ALGORITHM=COPY", length=68, parser_state=0x7f6736cd01f0, is_com_multi=false, is_next_command=false) at /data/src/10.4/sql/sql_parse.cc:8117

#12 0x000055e53a31beba in dispatch_command (command=COM_QUERY, thd=0x7f6720000b00, packet=0x7f672000a451 "ALTER TABLE t2 ADD FOREIGN KEY (a) REFERENCES t2 (b), ALGORITHM=COPY", packet_length=68, is_com_multi=false, is_next_command=false) at /data/src/10.4/sql/sql_parse.cc:1803

#13 0x000055e53a31a812 in do_command (thd=0x7f6720000b00) at /data/src/10.4/sql/sql_parse.cc:1356

#14 0x000055e53a489bdd in do_handle_one_connection (connect=0x55e53e6dd8a0) at /data/src/10.4/sql/sql_connect.cc:1398

#15 0x000055e53a48994e in handle_one_connection (arg=0x55e53e6dd8a0) at /data/src/10.4/sql/sql_connect.cc:1301

#16 0x000055e53a97285d in pfs_spawn_thread (arg=0x55e53e6f56f0) at /data/src/10.4/storage/perfschema/pfs.cc:1862

#17 0x00007f6742836494 in start_thread (arg=0x7f6736cd1700) at pthread_create.c:333

#18 0x00007f6740c1c93f in clone () from /lib/x86_64-linux-gnu/libc.so.6