"datetime >= coalesce(c1(NULL))" doesn't return expected NULL

mysql> create table t1(c1 timestamp(6) null default null);

Query OK, 0 rows affected (0.01 sec)

mysql> insert into t1 values(NULL);

Query OK, 1 row affected (0.00 sec)

mysql> select c1, '2016-06-13 20:00:00.000003' >= COALESCE( c1 ) from t1;

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

| c1   | '2016-06-13 20:00:00.000003' >= COALESCE( c1 ) |

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

| NULL |                                              1 |

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

1 row in set (0.00 sec)

mysql>

mysql> select version();

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

| version()       |

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

| 10.1.10-MariaDB |

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

1 row in set (0.00 sec)

drop table if exists t1;

create table t1(c1 timestamp(6) null default null);

insert into t1 values(NULL);

select c1, '2016-06-13 20:00:00.000003' >= COALESCE( c1 ) from t1;