sql_mode=ORACLE: using Oracle-style placeholders in direct query execution makes the server crash

SET sql_mode=DEFAULT; SELECT ?;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '?' at line 1

SET sql_mode=ORACLE; SELECT :a;

#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  0x0000555555e41753 in Item_param::val_str (this=0x7ffe6c0119a0, 

    str=0x7ffff4197bd0) at /home/bar/maria-git/server.10.2-ext/sql/item.cc:4085

#5  0x0000555555d36e0d in Type_handler::Item_send_str (

    this=0x5555571fcce8 <type_handler_varchar>, item=0x7ffe6c0119a0, protocol=

    0x7ffe6c001030, buf=0x7ffff4197ba0)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.cc:4963

#6  0x0000555555d39ac6 in Type_handler_string_result::Item_send (

    this=0x5555571fcce8 <type_handler_varchar>, item=0x7ffe6c0119a0, 

    protocol=0x7ffe6c001030, buf=0x7ffff4197ba0)

    at /home/bar/maria-git/server.10.2-ext/sql/sql_type.h:1593

#7  0x0000555555a97ea2 in Item::send (this=0x7ffe6c0119a0, 

    protocol=0x7ffe6c001030, buffer=0x7ffff4197ba0)