Update statement with join on Oracle

Question

I have this query but it's not working. Can you tell me the solution?

update temp_msisdn_saldo 
    set saldo=(
        select sum(saldo) from (
            select a.msisdn as mdn, a.saldo from temp_msisdn_saldo a 
            union all
            select b.mdn as mdn, b.saldo from temp_msisdn_saldo_yesterday b 
        ) z
    )
where msisdn=z.mdn