internal-server-error(500)

I use "org.jivesoftware.smack.Roster.createEntry(java.lang.String user, java.lang.String name, java.lang.String[] groups)" api to add an entry, it is throwing

internal-server-error(500)

     at org.jivesoftware.smack.Roster.createEntry(Roster.java:284)

source code follows:

    public void addUser(String userName, String name, String groupName) {

         getRoster().createEntry(userName, name, new String[] { groupName });

    }

    public Roster getRoster() {

        final Roster roster = xmppConnection.getConnection().getRoster();

        roster.setSubscriptionMode(Roster.SubscriptionMode.accept_all);

        roster.addRosterListener(rosterListener);

        return roster;

    }

Please help!