PHP Warning at __destruct()

[lisotton]

I'm using this piece of code:

$img = new SimpleImage($src);

// Resize image;
$img->best_fit($width, $height);

// Save image and output resized image.
$img->save($folder . '/' . $src)->output();

But all time it's log:
PHP Warning: imagedestroy(): 6 is not a valid Image resource in /home/ubuntu/sites/www.noticiasagricolas.com.br/includes/SimpleImage.php on line 57

For now, using is_resource inside of if statement of __destruct function solve the problem, but I think that this is not the best solution.