Change column type from timestamp WITHOUT time zone to timestamp WITH time zone

I found this ALTER COLUMN statement in the PostgreSQL 9.3 ALTER TABLE manual page:

ALTER TABLE audits
    ALTER COLUMN created_at SET DATA TYPE timestamp with time zone
    USING
        timestamp with time zone 'epoch' + created_at * interval '1 second';

I cannot seem to get this to work. I'm getting this error:

ERROR: operator does not exist: timestamp without time zone * interval
SQL state: 42883
Hint: No operator matches the given name and argument type(s).
You might need to add explicit type casts.

The ALTER TABLE statement looks very straight-foward. But I've tried all kinds of casts and converting column-types, but can not get this to work. What am I missing?