postgresql – How does ON CONFLICT work for composite foreign key columns?

I have problems with ON CONFLICT Does not work for foreign key columns where the foreign key is composed. Here is an example.

create table foreign_table
(
    id_a text    not null,
    id_b text    not null,
    id   integer not null
        constraint ft_pk
            primary key,
    constraint ft_a_b_key
        unique (id_a, id_b)
);

create table my_table
(
    id          integer,
    ftable_id_a text,
    ftable_id_b text,
    constraint my_table_a_b_fk
        foreign key (ftable_id_a, ftable_id_b) references 
    foreign_table (id_a, id_b)
);

With this query:

insert into tcell_test.my_table (id, ftable_id_a, ftable_id_b) 
    values (3, 'a3', 'b3') on conflict do nothing ;

where should I say 'a3' is not in foreign_tableI would expect that ON CONFLICT to handle the mistake.

Instead, I get the error:

(23503) ERROR: insert or update on table "my_table" 
   violates foreign key constraint "my_table_a_b_fk" 
Detail: Key (ftable_id_a, ftable_id_b)=(a3, b3) 
   is not present in table "foreign_table".

Is there a way to correct this? ON CONFLICT handles the mistake?