Cannot create SPATIAL/RTREE index on virtual column

Assume the table:

CREATE TABLE foo(

   x INTEGER NOT NULL,

   y INTEGER NOT NULL,

   pt POINT AS (POINTFROMTEXT(CONCAT('POINT ', x, ',', y, ')')))

);

The column pt is a virtual column, which it should be possible to create an index on, pre 10.2 it had to be persistent but that makes no difference here:

CREATE SPATIAL INDEX foo_ix1 ON foo(pt);

And this fails with:
ERROR 1252 (42000): All parts of a SPATIAL index must be NOT NULL