Getting random junk output from my simple syscall test

Using Ubuntu 18.04.3 LTS (GNU/Linux 5.0.21+ x86_64)

I created a simple syscall and am trying to pass input (the integer 5) through it by calling it in a c program and then see if I can get the input back out. I keep getting random junk as my output rather than my input.

Simple system call:

#include<linux/kernel.h>
#include<linux/init.h>
#include<linux/sched.h>
#include<linux/syscalls.h>
#include<linux/signal.h>
#include "tags.h"

asmlinkage int sys_get_tag(int pid)
{
  return pid;
}

my userspace test code:

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>
int main()
{
    int pid = syscall(335, 5); // 335 is the syscall number, 5 is the input to the syscall that I am trying to return
    printf("return: %d \n", pid);
    return 0;
}

my output:

return: 4980568

I get it to work when I return some integer in my syscall, like return 5; for example; so I know the syscall is being used.