Why nghttp2_session_mem_recv does not call any callback? (language: C, library: nghttp2)

Question

  ssize_t readlen;
  struct evbuffer *input = bufferevent_get_input(bev);
  size_t datalen = evbuffer_get_length(input);
  unsigned char *data = evbuffer_pullup(input, -1);

  readlen = nghttp2_session_mem_recv(session, data, datalen);
  if (readlen < 0) {
    printf("Fatal error: %s\n", nghttp2_strerror((int)readlen));
    return;
  }
  if (evbuffer_drain(input, (size_t)readlen) != 0) {
    printf("Fatal error: evbuffer_drain failed\n");
    return;
  }

nghttp2_session_mem_recv function does not call any callback even though it has all the callbacks set and the result returned by it is a valid one (return 130 and datalen is also 130).