Update Query Problem

the following update query...

UPDATE t_job_number
SET contract_id = 30
WHERE t_serial_number.serial_number_id = 78
AND t_serial_number.job_number_id =
t_job_number.job_number_id

yields the following chastisement...

ERROR:  missing FROM-clause entry for table
"t_serial_number"

can anyone point in the right direction?  requiring a
from clause in this update seems a little bizarre to
me - so i'm sure i've botched something up pretty
good.

tia...

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