Not all columns in super-aggregate rows in a SELECT ... WITH ROLLUP are NULL

CREATE TABLE t (a INT,b INT) ENGINE=INNODB;

insert into t (a, b) select seq / 20, seq from seq_1_to_100;

SELECT a, a, count(b) FROM t GROUP BY a WITH ROLLUP;

a	a	count(b)

0	0	9

1	1	20

2	2	20

3	3	20

4	4	20

5	5	11

NULL	5	100

MariaDB [test]> SELECT a,a+1, count(b) FROM t GROUP BY a WITH ROLLUP;

+------+------+----------+

| a    | a+1  | count(b) |

+------+------+----------+

|    0 |    1 |        9 |

|    1 |    2 |       20 |

|    2 |    3 |       20 |

|    3 |    4 |       20 |

|    4 |    5 |       20 |

|    5 |    6 |       11 |

| NULL | NULL |      100 |

+------+------+----------+

7 rows in set (0.005 sec)