Unable to get UPDATE ... FROM syntax correct

I have two tables:

Table "public.audit"
Column | Type | Modifiers
------------+----------------------+-----------
id | integer | (serial)
record_id | integer | not null
key | character varying |
(...)

Table "public.extension"
Column | Type | Modifiers
---------+------------+------------
id | integer | (serial)
number | integer |
(...)

The column "key" was recently added to the "audit" table, and I wish to
populate it with the value form the "number" column in the extension table.
The join between the tables is audit.record_id = extension.id

I tried:

UPDATE audit SET key = extension.number FROM extension WHERE audit.record_id
= extension.number;

But that returns saying "UPDATE 0"

However, doing:

SELECT audit.record_id, extension.number FROM audit, extension WHERE
audit.record_id = extension.id;

Works fine.

Can someone tell me what I'm doing wrong ?

Thanks,

GTG