Incorrect double free behavior

[ChAoSUnItY]

Using the code below to initialize buckets for hashmap and frees hashmap:

#include <stdlib.h>

typedef struct hashmap_node {
    char *key;
    void *val;
    struct hashmap_node *next;
} hashmap_node_t;

typedef struct {
    int size;
    hashmap_node_t **buckets;
} hashmap_t;

hashmap_t *hashmap_create(int size)
{
    hashmap_t *map = malloc(sizeof(hashmap_t));
    map->size = size;
    map->buckets = malloc(size * sizeof(hashmap_node_t *));

    for (int i = 0; i < map->size; i++)
        map->buckets[i] = 0;

    return map;
}

void hashmap_free(hashmap_t *map)
{
    for (int i = 0; i < map->size; i++) {
        for (hashmap_node_t *cur = map->buckets[i], *next; cur; cur = next) {
            next = cur->next;
            free(cur->key);
            free(cur->val);
            free(cur);
            cur = next;
        }
    }

    free(map->buckets);
    free(map);
}

int main()
{
    hashmap_t *map = hashmap_create(16);
    map->buckets[0] = malloc(sizeof(hashmap_node_t)); // Simulates put at first bucket
    map->buckets[0]->key = calloc(1, sizeof(char));
    map->buckets[0]->key = malloc(sizeof(int));
    map->buckets[0]->next = NULL;
    hashmap_free(map);
    return 0;
}

Compile with gcc will exit normally, while compile with shecc will cause double free, which is abnormal.