Floor of a BST with recursive function in C

Question

I'm trying to find the floor of a BST recursively. This is the code:

node_t* floor_impl(node_t *node, const void *key, comparator_t key_cmp)
{
    if(node == NULL)
    {
        return NULL;
    }

    if(key_cmp(key, node->key) == 0)
    {
        return node->key;
    }
    else if(key_cmp(key, node->key) < 0)
    {
        return floor_impl(node->left, key, key_cmp);
    }
    else
    {
        node_t *floor = floor_impl(node->right, key, key_cmp);
        if(floor == NULL) { return NULL; }
        return key_cmp(node->key, floor->key) >= 0 ? node : floor;
    }
}

This function must return a node, which contains the floor.

But the result is a segmentation fault error. I understood the theory behind the search of the floor, but not how to do it.