Erreur Notice: Undefined index

Notice: Undefined index: a in...

$action = 'new';               
if($_GET['a']) { $a = $_GET['a']; } 
if(isset($_GET['idclas'])) { $idclas = $_GET['idclas']; }

switch($a) {
	case 'new': $action = 'new';  break; 
	case 'ls' : $action = 'list'; break;
	case 'mod': $action = 'mod';  break;
	default   : $action = 'new';  break;
}