This issue might be related to MDEV-3819.

Geometry functions should be more restrictive and produce an error in case parameter is a valid geometry but has the wrong type.

According to the OGC standard definition the return value for a
function is NULL if the geometry is NULL (RETURNS NULL ON NULL).

The standard also defines the valid geometry types for functions:

7.2.9 SQL routines on type POINT

The SQL/MM ST_X, ST_Y, ST_Z and ST_M routines and all routines supported by type GEOMETRY shall be supported for geometries of type Point.

Example:

PostGIS

osm=# select st_x(GeomFromText('LINESTRING(1 1, 2 2)'));

ERROR:  Argument to X() must be a point

MariaDB

mysql> select st_x(GeomFromText('LINESTRING(1 1, 2 2)'));

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

| st_x(GeomFromText('LINESTRING(1 1, 2 2)')) |

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

|                                       NULL |

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

1 row in set (0.06 sec)

Affected functions (might be incomplete)

Geometry type POINT

• ST_X()
• ST_Y()
• ST_Z() (not implemented yet)

Geometry type CURVE (includes LINESTRING, LINE, LINEARRING)

• ST_StartPoint()
• ST_EndPoint()
• ST_IsRing()
• ST_Length()

Geometry type LINESTRING

• ST_NumPoints()
• ST_PointN()

Geometry type SURFACE (includes POLYGON, POLYHEDRALSURFACE)

• ST_Centroid()
• ST_Area()
• ST_PointOnSurface()

Geometry type POLYGON

• ST_ExteriorRing()
• ST_InteriorRingN()
• ST_NumInteriorRing()