mergejoin error message executing in 7.2

hi there,

I have a problem trying to execute a complex query that was designed in
7.3.2 to a machine running 7.2.x

this is the error message

FULL JOIN is only supported with mergejoinable join conditions

this is the source string for a jdbc prepared statement. (it has ordering
clause added at the end using string concatenation)
this string works fine in 7.3.2 but fails runtime in 7.2.

Does anyone know what the problem is?
I can't find the message on the net anywhere except in the 'C' source on
postgres's developer channel or some japanese site (sigh).

strSQL = "SELECT stock," +
" p.description," +
" p.stockgroup," +
" o.ordercurrent," +
" o.type," +
" s.quantity," +
" a.ordercurrent, " +
" a.type " +
" FROM " +
" (SELECT stock,ordercurrent,type,line " +
" FROM orderlines o " +
" WHERE o.theorder = ? " +
" AND (o.TYPE='P' OR o.TYPE='T') ) AS o" +
" FULL OUTER JOIN " +
" (SELECT DISTINCT ON (stock) stock,quantity " +
" FROM standingorders s " +
" WHERE s.account = ? " +
" AND s.dayno = ? " +
" AND s.delivery = ? "+
" AND commencedate <= ? " +
" ORDER BY stock, commencedate DESC) AS s " +
" USING (stock) " +
" FULL OUTER JOIN " +
" (SELECT stock,ordercurrent,type " +
" FROM orderlines ol " +
" WHERE ol.theorder = ? " +
" AND ol.TYPE<>'P' " +
" AND ol.TYPE<>'T' ) AS a " +
" USING (stock) " +
" INNER JOIN stockitems p ON (p.id=stock) " +
" WHERE p.status='N' " +
" ORDER BY ";

Any help greatly appreciated.

Regards,
Kevin