Updating with a subselect

Hi, I need to update a field from a table based in a count.

This is the query:


update    shops
set    itemsqty =
    (
    select     count(*)
    from     items i1
    join      shops s1 on i1.shopid = s1.shopid
    where   s1.shopid = s0.shopid
    )
from     shops s0


The problem I'm having is that all the shops are updated with the items quantity of the first shop.
I've tried using the count without a subselect, but PostgreSQL complains about it.

How can I do this?