Xmltype with math operators should end up with Error:Illegal parameter data types

MariaDB [test]> create table t ( a xmltype);

Query OK, 0 rows affected (0,051 sec)

MariaDB [test]> INSERT INTO t VALUES ('<a><b>1</b></a>'),('<x><y/></x>'),(NULL);

Query OK, 3 rows affected (0,018 sec)

Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> select a+1 from t;

ERROR 4078 (HY000): Illegal parameter data types xmltype and int for operation '+'

MariaDB [test]> select -a from t;

+------+

| -a   |

+------+

|   -0 |

|   -0 |

| NULL |

+------+

3 rows in set, 2 warnings (0,002 sec)

Warning (Code 1292): Truncated incorrect DOUBLE value: '<a><b>1</b></a>'

Warning (Code 1292): Truncated incorrect DOUBLE value: '<x><y/></x>'

MariaDB [test]> select +a from t;

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

| a               |

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

| <a><b>1</b></a> |

| <x><y/></x>     |

| NULL            |

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

3 rows in set (0,002 sec)