Update with join ignores where clause - updates all rows

This WHERE clause finds a single row that has the same year,month,day,hour =
in another table.
It correctly counts 1 row.

SELECT count(*) FROM
       dw.prints_by_hour_work w  , dw.prints_by_hour h
WHERE
              w.year  =3D h.year
       and  w.month =3D h.month
       and  w.day   =3D h.day
       and  w.hour  =3D h.hour


The same join here, updates every row in the table which is incorrect.


update
       dw.prints_by_hour
set
       count =3D h.count + w.count
from
       dw.prints_by_hour_work w  , dw.prints_by_hour h
WHERE
              w.year  =3D h.year
       and  w.month =3D h.month
       and  w.day   =3D h.day
       and  w.hour  =3D h.hour

Is the join supported for UPDATE ?

If its supported then I can send the create statements etc so you can repro=
duce.