COPY TO (query) allows no function arguments

Hello all,

it is as simple as the subject says:

The COPY TO (query) statement does not allow the passing of function arguments - is this by intent or a bug?

Minimal working example:

CREATE OR REPLACE FUNCTION a(myparam text)
RETURNS void LANGUAGE plpgsql AS $func$
BEGIN
COPY (SELECT myparam) TO '/tmp/a.txt';
END;
$func$

SELECT a('b');

... will fail with

SQL-Fehler [42703]: FEHLER: Spalte »myparam« existiert nicht
Wobei: PL/pgSQL-Funktion a(text) Zeile 3 bei SQL-Anweisung

Sorry, my installation is German. This roughly translates to "column myparam does not exist".

I am using Postgres 13.

Regards,
Franz-Josef Färber