DROP TABLE IF EXISTS t1;
CREATE TABLE t1
(
  year    INT NOT NULL,
  product VARCHAR(32) NOT NULL,
  profit  INT
);
INSERT INTO t1 VALUES ('2001','car',101);
INSERT INTO t1 VALUES ('2001','gas',102);
INSERT INTO t1 VALUES ('2001','toy',103);
INSERT INTO t1 VALUES ('2002','car',201);
INSERT INTO t1 VALUES ('2002','gas',202);
INSERT INTO t1 VALUES ('2002','toy',203);
SET sql_mode=DEFAULT;
PREPARE stmt FROM "SELECT DATE'2001-00-00' AS c,year, SUM(profit) FROM t1 GROUP BY c,year WITH ROLLUP";
EXECUTE stmt;
SET sql_mode='no_zero_in_date';
EXECUTE stmt;

(gdb) where
#0  0x00000000005a6642 in String::length (this=0x0)
    at /home/bar/maria-bzr/maria-10.0/sql/sql_string.h:130
#1  0x00000000006ed824 in sortcmp (s=0x7fff70c23e60, t=0x0, 
    cs=0x173de60 <my_charset_latin1>)
    at /home/bar/maria-bzr/maria-10.0/sql/sql_string.cc:726
#2  0x000000000086acac in Cached_item_str::cmp (this=0x7fff70c23e40)
    at /home/bar/maria-bzr/maria-10.0/sql/item_buff.cc:94
#3  0x00000000006b7c62 in test_if_group_changed (list=...)
    at /home/bar/maria-bzr/maria-10.0/sql/sql_select.cc:21647
#4  0x00000000006b0c20 in end_send_group (join=0x7fff70c221b8, 
    join_tab=0x7fff70c235c8, end_of_records=false)
    at /home/bar/maria-bzr/maria-10.0/sql/sql_select.cc:18757
#5  0x00000000006ae11e in evaluate_join_record (join=0x7fff70c221b8, 
    join_tab=0x7fff70c232a0, error=0)
    at /home/bar/maria-bzr/maria-10.0/sql/sql_select.cc:17689
...
(gdb) p this
$1 = (const String * const) 0x0

+------------+------+-------------+
| c          | year | SUM(profit) |
+------------+------+-------------+
| 2001-00-00 | 2001 |         306 |
| 2001-00-00 | 2002 |         606 |
| 2001-00-00 | NULL |         912 |
| NULL       | NULL |         912 |
+------------+------+-------------+

SET sql_mode=DEFAULT;
PREPARE stmt FROM "select isnull(DATE'2001-00-00'), DATE'2001-00-00'";
SET sql_mode='no_zero_in_date';
EXECUTE stmt;

+--------------------------+------------------+
| isnull(DATE'2001-00-00') | DATE'2001-00-00' |
+--------------------------+------------------+
|                        0 | NULL             |
+--------------------------+------------------+
1 row in set, 1 warning (0.00 sec)