nativecall-c-Mu

/* this is C code test.c */

#include <math.h>
#include <stdio.h>

void custom() {
    printf("Hello world !");
}

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# Now calling via nativecall in raku
use NativeCall;

sub custom() is native('libtest') { * };

say custom()

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I get this output:

(Mu)
Hello world !

Why is this (Mu). How to stop it from appearing ?