NativeCall callback

NC-callback.p6

c code, compiler to libgetint.so :
#include <stdio.h>

int getint()
{
	return 42;
}

int setint(int* ptr, int(*get)())
{
	*ptr = get();
	return *ptr;
}

perl6 code - Rakudo version 2017.02 built on MoarVM version 2017.02:
#!/usr/bin/env perl6

use v6;
use NativeCall;

sub getint() returns int32 is native('./getint') { * }

sub setint(int32 is rw, &cb (--> int32)) returns int32 is native('./getint') { * }

my int32 $i = 89;

sub x() {
	return 42;
}

say setint($i, &getint); # rakudo complain - Cannot invoke object with invocation handler in this context
say setint($i, &x); # ok - 42

 # ? Why use getint as callback throw a exception ? Is my code incorrect ?