cast problem in Postgresql 9.0.1

I migrated my DB from Postgresql 8.1 to Postgresql 9.0.1.

I have a table "testtab"
\d testtab
id int,
hours varchar

When I execute the following:
select sum(hours) from testtab
I get cast error.

Then,

I created following IMPLICIT CAST functions in my DB =>

CREATE FUNCTION pg_catalog.integer(varchar) RETURNS int4 STRICT IMMUTABLE
LANGUAGE SQL AS 'SELECT int4in(varcharout($1));';
CREATE CAST (varchar AS integer) WITH FUNCTION pg_catalog.integer(varchar)
AS IMPLICIT;

CREATE FUNCTION pg_catalog.smallint(varchar) RETURNS smallint STRICT
IMMUTABLE LANGUAGE SQL AS 'SELECT int2in(varcharout($1));';
CREATE CAST (varchar AS smallint) WITH FUNCTION pg_catalog.smallint(varchar)
AS IMPLICIT;

Now, the above query works, but
SELECT COALESCE(hours,0) from testtab
failed.

Any idea why?