Could somebody please help me out with UPDATE-FROM ?

My table 'Rates' has a structure:

currency_id_1 int
currency_id_2 int
rate float
flag boolean

I'm trying to recalculate rates for all the entries marked by flag=1
using the rates from THE SAME table:

update Rates set rate = t1.rate * t2.rate from Rates t1, Rates t2
where currency_id_1 = t1.currency_id_1
and t1.currency_id_2 = t2.currency_id_1
and t2.currency_id_2 = currency_id_2
and flag = 1

Sybase ASE 12.5 complains with:

Ambiguous column name currency_id_1

What a problem here? How to bypass that (without temporary tables)?

Thanks,
Aleksey