`getopt` with atoi causes segmentation fault

#include <stdio.h>
#include <getopt.h>
#include <stdlib.h>


#define PORT 12344

int main(int argc, char** argv) {
    int opt;
    int server_fd, client_fd, epoll_fd;
    int child_pid;
    int port = PORT;
    char* file = NULL;

    /* Argparse */
    static struct option long_options[] = {
        {"file", required_argument, 0, 'f'},
        {"port", optional_argument, 0, 'p'},
        {"help", no_argument, 0, 'h'},
        {NULL, 0, NULL, 0}
    };


    while ((opt = getopt_long(argc, argv, "f:p:h", long_options, NULL)) != -1) {
        switch (opt) {
            case 'f':
                printf("file is: %s\n", optarg);
                break;
            case 'p':
                port = atoi(optarg);
                // since we will be listening on port > 1024 anyway, 0 is unused
                if (!port) {
                    printf("not a valid port\n");
                    exit(1);
                }
                break;
            case 'h':
                printf("help message\n");
                exit(0);
            default:
                break;
        }
    }

    printf("end\n");
}

I am trying to build an application based on TCP. I want the program to take two arguments, a file parameter which specifies the configuration file, a number which is basically the port number to listen on The oher arguments work just fine. But, whenever I try to use the --port ABC option, the program crashes with a Segmentation fault.

What am I missing here?