COALESCE in ORDER BY... - Mailing list pgsql-sql

Why this doesn't run on postrgreSQL 7.3.1 ?

www=> SELECT 10 AS first ORDER BY first DESC;                            first
-------   10
(1 row)

www=> SELECT 10 AS first ORDER BY COALESCE(first,0) DESC;
ERROR:  Attribute "first" not found

Thanks in advance..