Syscall argument in kprobe with wrong value libbpf

I'm trying to use libbpf to trace calls to the kill syscall. Here is my eBPF program:

SEC("kprobe/__x64_sys_kill")
int BPF_KPROBE(__x64_sys_kill, pid_t pid, int sig)
{
    bpf_printk("Pid = %i\n", pid);

    return 0;
}

But for some reason, when I try to read the pid argument, the value is negative. But when using strace on the kill command the value of pid is positive.

$ ping 8.8.8.8 > /dev/null &
[1] 87120
$ strace kill -9 $(pidof ping)
...
kill(87120, SIGKILL)                    = 0
...
[1]+  Killed                  ping 8.8.8.8 > /dev/null

Logs:

bash-83960   [001] d... 42409.690336: bpf_trace_printk: Pid = -1060765864

I can't figure out why the value of the pid argument insde the eBPF program is not the same as the one given by the urserland process