psql \COPY accepts multiple NULL AS

Greetings,

psql seems to accept syntax which would be rejected by the server,
which seems a bit odd:

sfrost*=# \copy billing_data from ../BillingSamplePricerFile.csv
with csv header quote as '"' null as 'abc' null as '123'
ERROR: invalid input syntax for integer: ""

vs.

sfrost=# copy billing_data from stdin
with csv header quote as '"' null as 'abc' null as '123';
ERROR: conflicting or redundant options

Now, honestly, I'd like to see support for multiple 'NULL AS' strings,
but till that's added it'd probably be best if psql error'd on this
like the backend does rather than accepting it and then not doing it.

Thanks!

Stephen