use datagram_flow_id as index in `conn->datagram_flows`
@@ -1445,7 +1445,7 @@ static void establish_tunnel(h2o_req_t *req, h2o_tunnel_t *tunnel, uint64_t idle
/* register to the map */
struct st_h2o_http3_server_conn_t *conn = get_conn(stream);
int r;
- khiter_t iter = kh_put(stream, conn->datagram_flows, stream->quic->stream_id, &r);
+ khiter_t iter = kh_put(stream, conn->datagram_flows, stream->tunnel->datagram_flow_id, &r);
assert(iter != kh_end(conn->datagram_flows));
kh_val(conn->datagram_flows, iter) = stream;
/* set the callback */
