Trigger Critique

Here is a trigger I wrote to check for duplicate taxid's in my producer table. Duplicate taxid's with an '' value are allowed. I'm green when it comes to trigger coding so would anyone care to review my code and reply if there's a more efficient way to implement the trigger or handle errors.

Thanks PLJ

create trigger dbo.tib_producer on dbo.producer for insert as
begin
declare
@numrows int,
@errno int,
@errmsg varchar(255)
select @numrows = @@rowcount
if @numrows = 0
return
/* Check for duplicate tax_id if inserted is not null */
if update(tax_id)
begin
if exists(select tax_id
from producer
where tax_id in (select tax_id
from inserted)
and tax_id <> ''
group by tax_id having count(*)>1)
begin
select @errno = 50002,
@errmsg = 'Tax ID already Exists'
goto error
end

end
return
/* Errors handling */
error:
raiserror @errno @errmsg
rollback transaction
end