PTRACE_SETOPTIONS with PTRACE_O_EXITKILL not working: invalid argument

Question

I'm learning how to use ptrace and wrote some code to test it out. However, I am getting a problem when I am trying to use the PTRACE_SETOPTIONS request to set the PTRACE_O_EXITKILL option to kill the tracee process if the tracer process terminates.

For reference, I am using Windows Subsystem for Linux's Kali Linux, running kernel version 4.4.0.

void child_code();
void parent_code(pid_t pid);

int main(const int argc, char *argv[])
{
    pid_t pid;
    switch (pid = fork())
    {
        case -1:
            perror("fork");
            break;
        case 0:
            child_code();
            break;
        default: //parent code
            parent_code(pid);
    }

    return 0;
}

void parent_code(pid_t pid)
{
    printf("Parent code\n");
    int status;
    if (wait(&status) == -1)
    {
        perror("parent wait one");
    }
    printf("Finished waiting\n");
    printf("PTRACE_O_EXITKILL is %x\n", PTRACE_O_EXITKILL); //0x100000
    printf("PTRACE_O_TRACEEXEC is %x\n", PTRACE_O_TRACEEXEC); //0x10
    printf("PTRACE_O_TRACEFORK is %x\n", PTRACE_O_TRACEFORK); //0x2
    printf("My PID is     %d\n", getpid());
    err_wrap(
        ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_EXITKILL), 
        0, "ptrace-set-options");
    //This is returning EINVAL error and prints "Invalid argument" and I don't know why.

}


void child_code() {
    printf("Child code\n");
    printf("Parent is:   %d\n", getppid());
    err_wrap(ptrace(PTRACE_TRACEME, 0,0,0), 0, "ptrace-traceme");
    err_wrap(raise(SIGSTOP), 0, "raise");

    unsigned int pleb = 0xffbbcde8;
    int x = 0;
    printf("Hello WoRld\n");

    printf("Pleb is:     %llx\n", pleb);
    printf("x is:        %d\n", x);
}

err_wrap is defined here

void err_wrap(const int ret, const int success, const char *msg)
{
    if (ret != success)
    {
        perror(msg);
    }
}

When I call PTRACE_SETOPTIONS with PTRACE_O_EXITKILL, it's saying invalid argument, but I do not know why. When I try other options like PTRACE_O_TRACEEXEC, it works fine. I haven't tried all other options though.

I can't understand why it's not working. As PTRACE_O_EXITKILL is defined, and it's not a macro/variable that I've made, I would presume that the ptrace would recognize it, but for some reason it doesn't. I printed it out and it matches the value that I see in ptrace source code (0x100000).

Any help would be much appreciated, thanks.