how to extract the raw value in a subdocument in mongo's libbson c library?

Question

consider the mongo document: {"key1": "value1", "key2": {"subkey1": "subvalue1"}}, this can be encoded (using python bson module like: bson.encode(DATA)) to the following uint8_t array: [56, 0, 0, 0, 2, 107, 101, 121, 49, 0, 7, 0, 0, 0, 118, 97, 108, 117, 101, 49, 0, 3, 107, 101, 121, 50, 0, 28, 0, 0, 0, 2, 115, 117, 98, 107, 101, 121, 49, 0, 10, 0, 0, 0, 115, 117, 98, 118, 97, 108, 117, 101, 49, 0, 0, 0]. Now, I use this array to initialize the bson_t struct and use the iterator to find the subdocument for key2.

Here I want to create another bson_t document from this. I tried using the bson_iter_document() method but it gives me precondition failed: document error. Maybe i'm not using it right. Is there another way to do it properly?

void test_bson(){
    uint8_t raw[] = {56, 0, 0, 0, 2, 107, 101, 121, 49, 0, 7, 0, 0, 0, 118, 97, 108, 117, 101, 49, 0, 3, 107, 101, 121, 50, 0, 28, 0, 0, 0, 2, 115, 117, 98, 107, 101, 121, 49, 0, 10, 0, 0, 0, 115, 117, 98, 118, 97, 108, 117, 101, 49, 0, 0, 0};
    bson_t *bson;
    bson = bson_new_from_data(raw, 56);

    bson_iter_t iter;
    bson_iter_init(&iter, bson);

    if (bson_iter_find(&iter, "key2")){
        printf("found. %d\n", bson_iter_type(&iter));
        const uint8_t **subdocument;
        uint32_t subdoclen = 56;
        bson_iter_document (&iter, &subdoclen, subdocument);
    }

    bson_free(bson);
}