ERROR: unexpected SELECT query in exec_stmt_execsql()

Hi. I am attempting to create a function to do some selects and return
some information. Pretty straight forward. No matter what I do, I always
get this error. Here is my stored function and how I am calling it...

create function sp_list_categories(int4) returns varchar as '
DECLARE
postid ALIAS FOR $1;
parentid int4;
BEGIN

SELECT * FROM category WHERE id = postid;
parentid := SELECT parent FROM category WHERE id = postid;

IF parentid ISNULL THEN
RETURN
ELSE
SELECT sp_list_categories(parentid);
RETURN name;
END IF;

END;
' LANGUAGE 'plpgsql';

jimmydbase=# select sp_list_categories(20);
ERROR: unexpected SELECT query in exec_stmt_execsql()

Any and all help would be appreciated!

Branden.