PG7.4 ordering operator

Testing postgis support in PG7.4 (2003-11-11)
I've encountered to this problem:
       ERROR:  could not identify an ordering operator for type geometry       HINT:  Use an explicit ordering operator
ormodify the query.
 

Whenever I issue one of these commands:
gis=# select the_geom from table1 UNION select the_geom from table2;gis=# select DISTINCT the_geom from table1;gis=#
selectthe_geom from table1 ORDER BY the_geom;
 

Operators '<', '>', '=' are available:
 oprname | leftoperand | rightoperand---------+-------------+-------------- <<      | geometry    | geometry &<      |
geometry   | geometry &&      | geometry    | geometry &>      | geometry    | geometry >>      | geometry    |
geometry~=      | geometry    | geometry @       | geometry    | geometry ~       | geometry    | geometry =       |
geometry   | geometry <       | geometry    | geometry >       | geometry    | geometry(11 rows)
 

Previous PG versions does not show this problem.
Any hint on what might be missing ?

--strk;