prepare in a do loop

in psql, with a postgres 12.5 db on a centos 7 intel:
I do create a function named randname() returning a varchar, and a table matable with a column prenom varchar(50). then
postgres=# prepare moninsert(varchar) as
postgres-# insert into matable(prenoms) values($1);
PREPARE

I test it:
postgres=# execute moninsert(randname());
INSERT 0 1

up to now, everything fine. then:
do $$ begin for counter in 1..1000000 loop execute moninsert(randname());end loop;end;$$;
ERREUR:  la fonction moninsert(character varying) n'existe pas
LIGNE 1 : SELECT moninsert(randname())

someone can explain ?

thanks

(its a french db, so error message in french :-)