IFNULL() does not work well WITH ROLLUP

MariaDB [test]> CREATE OR REPLACE TABLE t (a INT, b INT) ENGINE = InnoDB;

Query OK, 0 rows affected (0.34 sec)

MariaDB [test]> INSERT INTO t VALUES

    ->         (1, 1),

    ->         (1, 2),

    ->         (2, 1),

    ->         (NULL, 1);

Query OK, 4 rows affected (0.09 sec)

Records: 4  Duplicates: 0  Warnings: 0

MariaDB [test]> SELECT a, COUNT(*)

    ->         FROM t

    ->         GROUP BY a

    ->         WITH ROLLUP;

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

| a    | COUNT(*) |

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

| NULL |        1 |

|    1 |        2 |

|    2 |        1 |

| NULL |        4 |

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

4 rows in set (0.00 sec)

MariaDB [test]> SELECT IFNULL(a, 1), COUNT(*)

    ->         FROM t

    ->         GROUP BY a

    ->         WITH ROLLUP;

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

| IFNULL(a, 1) | COUNT(*) |

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

|            1 |        1 |

|            1 |        2 |

|            2 |        1 |

|            1 |        4 |

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

4 rows in set (0.00 sec)