Update field to a column from another table

I've got two tables:

- ja_jobs
- junk.ja_jobs_23856

I need to update the null column ja_jobs.time_job with the data from the
table junk.ja_jobs_23856

So I'm doing:

>
> UPDATE public.ja_jobs AS b
> SET time_job = a.time_job
> FROM junk.ja_jobs_23856 AS a
> WHERE a.id = b.id
> AND a.clientid = b.clientid;

But it's now working... I'm using PostgreSQL 9.2

Do you guys have an idea why?

cheers;
Lucas