ST_PointOnSurface returns NULL with Polygon in some case

SET @g1 = ST_GeomFromText('POLYGON((0 0, 0 1,1 1, 1 0, 0 0))');

SET @g2 = ST_GeomFromText('POLYGON((-1 0, 1 2, 1 0, -1 0))');

SELECT ST_WITHIN(@g1, @g2);

-- result:{t}

SELECT ST_ASTEXT(ST_POINTONSURFACE(@g1));

-- result:{POINT(0.5 0.5)}

SELECT ST_ASTEXT(ST_POINTONSURFACE(@g2));

-- result:{NULL}