Table variables - bug or usage issue

When using table variables like this

declare @tab table

(

    PK char(8),

    Currency char(3)

)

update @tab

set Currency = s.Currency

from @tab p , s

where p.PK = s.PK

I get the message

      Ambiguous column name Currency

I've tried

     set p.Currency = s.Currency

Also,

     select p.Currency c1, s.Currency c2

     from @tab, s

gives 2 error messages

     Ambiguous column name Currency

What is going on ?

Is there a workaround.