Array / lister error

Jeg får en fejl så snart denne timer udføres.
(Access Violation)
Eneste undtagelse er hvis  lstUsers.Items.Count=0



procedure TfrmServidor.Timer1Timer(Sender: TObject);
var
retorno : TMsg;
i : integer;
begin
{
  for i:=0 to length(A) do
Memo1.Lines.add(\'A\'+A[i]+\'B\'+B[i]);
}




if lstUsers.Items.Count>0 then
begin
  SetLength(A, lstUsers.Items.Count);
  SetLength(B, lstUsers.Items.Count);

          for i:=0 to lstUsers.Items.Count-1 do
          //Array A contains usernames
          //Array B contains a integer for ping result (0 until answered)
        begin
            memo1.Lines.Add(\'Pinging user \'+lstUsers.Items.Strings[i]);

            retorno.Tipo := MSG_PING;
            retorno.destinatario := lstUsers.Items.Strings[i];
            sscChat.Socket.Connections[i].SendBuf( retorno, SizeOf(TMsg) );
        A[i]:=lstUsers.Items.Strings[i];
        B[i]:=0;
        end;
end;
end;