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!