User defined cast creation

Hi friends,

When I try to create a function which is used in user defined type cast I get the following error,

CREATE FUNCTION pg_catalog.text1(integer) RETURNS text STRICT IMMUTABLE LANGUAGE SQL AS 'SELECT textin(int4out($1));';
ERROR: permission denied for schema pg_catalog

I am not trying to create this function as postgres user, but as 'user1'. I need to create a user defined cast which uses this function for this particular user. I could create functions which doesn't use pg_catalog schema as same user.
What's the issue here? How can I give permission for this user1 or what's the right way to create user defined cast for a particular user?

Thanks in Advance for your help,

Arun~