PG Version 8.1

Hi, i use pg version 8.0 and i have this case:

UPDATE tbpeople
SET dt_update = '11/22/2005'
WHERE tbdoctor.id_people = tbpeople.id

But now, in version 8.1, this instrution give me an error:
[ERROR:  missing FROM-clause entry for table "tbdoctor"]

Who can i do now to get the same result?

Thanks...
Diego Ziquinatti