How to upsert with serial primary key - postgresql

Question

I have a stored procedure to perform an upsert. However the conflict condition never runs, passing an existing ID always causes it to create a new record.
create or replace function upsert_email(v_id bigint, v_subject character varying)
returns bigint
language plpgsql
as $$
declare
v_id bigint;
begin
insert into emails
(id, subject)
values (coalesce(v_id, (select nextval('serial'))), v_subject)
on conflict(id)
do update set (subject) = (v_subject) where emails.id = v_id
returning id into v_id;
return v_id;
end;
$$;
When running select upsert_email(6958500, 'subject'); which is a record that exists, it always creates a new record instead.
I have already looked at: Upsert/on conflict with serial primary key which is the most similar question and is what my SQL is modeled on, however I haven't been able to get it to work.

Answer 1

Wow, idiot of the year award goes to me.
I'm taking a parameter called v_id, then immediately overwrite it in the declare with a v_id; They should be named 2 different things:
create or replace function upsert_email(v_id bigint, v_subject character varying)
returns bigint
language plpgsql
as $$
declare
v_return_id bigint;
begin
insert into emails
(id, subject)
values (coalesce(v_id, (select nextval('serial'))), v_subject)
on conflict(id)
do update set (subject) = (v_subject) where emails.id = v_id
returning id into v_return_id;
return v_return_id;
end;
$$;