[postgis-users] envelope returns points

[Benedetto Porfidia]

Hi,

I'm tryin this query

SELECT DISTINCT ON (mytable.my_id) mytable.my_id, 
asText(envelope(mytable.the_geom)) AS the_geom
    FROM mytable
   ORDER BY mytable.my_id;

but envelope function returns points instaed of polygons.
My geometries are all points.
I'm working on postgres 9.0 and postgis 1.5 upgraded from vers. 8.2 and 
1.4(?)on win32; with old version but same table, the query works fine 
(returns polygon).

Any suggestion is welcome

Benedetto