Android : Concurrent modification exception

My following code is throwing concurrent modification exception and I am not able to solve it. Help!

  {
    long now = new Date().getTime();
    Set<String> keyset = requests.keySet();
    Iterator<String> iterator = keyset.iterator();

    while (iterator.hasNext()) {
        String packetId = iterator.next();
        RequestHolder entry = requests.get(packetId);
        if (entry.isExpired(now)) {
            entry.getListener().onTimeout(packetId);
            iterator.remove();
        }
    }
  }