GIS: Functions return NULL instead of specified -1 for NULL arguments

MariaDB [test]> select ST_IsRing(NULL);

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

| ST_IsRing(NULL) |

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

|            NULL |

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

1 row in set (0.00 sec)

etc.

Specification in the email said:

IsRing(ln)
The return type is Integer, with a return value of 1 for
TRUE, 0 for FALSE, and -1 for UNKNOWN
corresponding to a function invocation on NULL

etc.

Possibly it's just a documentation issue. PostgreSQL also returns NULL.