ERROR: could not determine which collation to use for stringcomparison

create table person(   id serial primary key,   firstname varchar,   lastname varchar
);

insert into person(firstname, lastname) values ('Andreas', 'Krogh');

CREATE OR REPLACE FUNCTION concat_lower(varchar, varchar) RETURNS varchar AS
$$
SELECT nullif(lower(coalesce($1, '')) || lower(coalesce($2, '')), '')
$$ LANGUAGE SQL
    IMMUTABLE;

select * from person pers ORDER BY concat_lower(pers.firstname, pers.lastname) ASC;