How does the undefined system call return -1?

I defined the "helloworld" system call in my Linux kernel and recompiled it. System call code:

#include<linux/kernel.h>
#include<linux/init.h>
#include<linux/sched.h>
#include<linux/syscalls.h>
#include "processInfo.h"
asmlinkage long sys_listProcessInfo(void)
{
    printk("Hello World. My new syscall..FOSS Lab!\n");
    return 0;
}

      

        
        
        
      

    

But when I call this system call from the same operating system with a different kernel version that does not include this system call using the code below:

#include<stdio.h>
#include<linux/kernel.h>
#include<sys/syscall.h>
#include<unistd.h>
int main()
{
    long int var = syscall(326);
    printf("Returning: %ld\n",var);
    return 0;
}

      

        
        
        
      

    

The variable var

gets the value -1. I would like to know how it var

gets -1 instead of displaying an error.