function fetch_all of the objetvo mysqli does not give results

I have the following code.

class IQTCulturaBD{
protected $mysqli;
const LOCALHOST = 'localhost';
const USER = 'xxxx';
const PASSWORD = 'xxxxxxx';
const DATABASE = 'fffff';
/**
 * Constructor de clase
 */
public function __construct() {
    try{
        //conexion a base de datos
        $this->mysqli = new mysqli(self::LOCALHOST, self::USER, self::PASSWORD, self::DATABASE);
    }catch (mysqli_sql_exception $e){
        //Si no se puede realizar la conexion
        http_response_code(500);
        exit;
    }
}

public function geElemento($id=0){
        $stmt = $this->mysqli->prepare("SELECT * FROM elemento WHERE id=?");
        $stmt->bind_param('s', $id);
        $stmt->execute();
        $result = $stmt->get_result();
        $elemento = $result->fetch_all(MYSQLI_ASSOC);
        $stmt->close();
        return $elemento;
}
public function getElementos(){
        $sql = "SELECT id,nombre,encabezado,latitud,longitud FROM elemento";
        $result = $this->mysqli->query($sql);                                
        $elementos = $result->fetch_all(MYSQLI_ASSOC);                
        $result->close();
        return $elementos;
}

}

but the function $ elements = $ result-> fetch_all (MYSQLI_ASSOC); It does not work for me, it does not give me an error either, the PHP version is 5.5.12