Trigger problem, record "new" is not assigned yet

Hello.

I'm probably doing some very basic error here, but I get

ERROR: record "new" is not assigned yet
The tuple structure of a not-yet-assigned record is indeterminate.

when I try this small example

create table foo(x int);

create or replace function trigger_foo() returns trigger language plpgsql as $$
declare v_i integer;
begin
select count(1) into v_i from foo;
if new.x >18 then
raise exception 'error';
else
return null;
end if;
end; $$;

CREATE TRIGGER trigger_foo AFTER INSERT ON foo for each row EXECUTE
PROCEDURE trigger_foo();

insert into foo (x) values (15);

And the table foo only have 14 lines right now.

Can someone help me spot the error? :-)