Null is inserted by INSERT SELECT MAX () to the same table.

CREATE TABLE t1(id1 int, id2 int, PRIMARY KEY (id1,id2)) ENGINE=InnoDB;

INSERT t1 VALUES(1, 1);

INSERT t1 SELECT id1, MAX(id2)+1 FROM t1 WHERE id1=1;

mysql> CREATE TABLE t1(id1 int, id2 int, PRIMARY KEY (id1,id2));

Query OK, 0 rows affected (0.01 sec)

mysql> INSERT t1 VALUES(1, 1);

Query OK, 1 row affected (0.00 sec)

mysql> INSERT t1 SELECT id1, MAX(id2)+1 FROM t1 WHERE id1=1;

ERROR 1048 (23000): Column 'id2' cannot be null