Encountering SIGSEGV when parsing multiple rule sets in parallel about modsecurity HOT 4 OPEN

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <modsecurity/modsecurity.h>
#include <modsecurity/rules_set.h>

void *create_rules(void *arg)
{
    const char *plain_rules = arg;

    const char *err = NULL;

    RulesSet *rules;

    rules = msc_create_rules_set();
    if (rules == NULL)
    {
        return NULL;
    }

    int retval = msc_rules_add(rules, plain_rules, &err);

    if (retval < 0)
    {
        fprintf(stderr, "Error - msc_rules_add(): %s\n", err);
        exit(EXIT_FAILURE);
    }

    return NULL;
}

int main()
{
    pthread_t thread1, thread2;
    int iret1, iret2;

    const char *plain_rules_1 = "SecRuleEngine On\n";
    const char *plain_rules_2 = "SecRuleEngine On\n";

    iret1 = pthread_create(&thread1, NULL, create_rules, (void *)plain_rules_2);
    if (iret1)
    {
        fprintf(stderr, "Error - pthread_create() return code: %d\n", iret1);
        exit(EXIT_FAILURE);
    }

    iret2 = pthread_create(&thread2, NULL, create_rules, (void *)plain_rules_1);
    if (iret2)
    {
        fprintf(stderr, "Error - pthread_create() return code: %d\n", iret2);
        exit(EXIT_FAILURE);
    }

    // Wait till threads are complete before main continues.
    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);

    return 0;
}

pthread_mutex_t lock;

void *create_rules(void *arg)
{
    const char *plain_rules = arg;

    const char *err = NULL;

    RulesSet *rules;

    rules = msc_create_rules_set();
    if (rules == NULL)
    {
        return NULL;
    }

    pthread_mutex_lock(&lock);
    int retval = msc_rules_add(rules, plain_rules, &err);
    pthread_mutex_unlock(&lock);

    if (retval < 0)
    {
        fprintf(stderr, "Error - msc_rules_add(): %s\n", err);
        exit(EXIT_FAILURE);
    }
    else {
        fprintf(stdout, "Done - msc_rules_add(): %s\n", err);
    }

    return NULL;
}