getIterator() on a non-object

<?PHP
class Clases implements Iterator
{
    private $clases;
 private $puntero;
    public function addClase (Clase $clase)
    {
        $this->clases[] = new $clase;
    }
 
 public function getClase ($id)
    {
// ich denke hier ist den error//
 $iterator = $this ->clases->getIterator();  
  return "ok ".$id;
  while($iterator->valid())  
  {  
   $clase = $iterator->current();  
 
   if($clase->getId() == $id)  
   {  
    return $clase;  
   }  
 
   $iterator->next(); 
  }
        return null;
    }
    public function current ()
    {
        if (! $this->valid())
            return false;
        if (empty($this->clases[$this->puntero]))
            return array();
        return $this->clases[$this->puntero];
    }
    public function key()
    {
        return $this->puntero;
    }
    public function next()
    {
        return ++ $this->puntero;
    }
    public function rewind()
    {
        $this->puntero = 0;
    }
    public function valid()
    {
  return $this->puntero !== false;
    }
    public function count()
    {
        return count($this->clases);
    }
/*
    public function load($nombre)
    {
        $data = simplexml_load_file("data.xml");
        foreach ($data->{$nombre} as $registro) {
            foreach ($registro as $persona) {
                $alumno = new Alumno($persona->id, $persona->nombre, $persona->apellidos);
                $this->addAlumno($alumno);
            }
        }
    }
*/
}
?>