Upload a file to S3 with AWS SDK (PHP) - Cant upload a non-object

I have the following code, which is working correctly up until the point of using the upload function. I then get the error -

Call to a member function upload() on a non-object.

I understand what the error means, but I do not why? Any ideas. Googling has come up with zilch.

require('/libs/s3/aws-autoloader.php');

use Aws\S3\S3Client;
use Aws\S3\Exception\S3Exception;

// Instantiate an S3 client
$s3Client = S3Client::factory(array(
    'profile' => 'default',
));

    try {
    $resource = fopen(DRUPAL_ROOT . '/LICENSE.txt', 'r');  

    $s3->upload('be-production', 'assets', $resource, 'public-read');
} catch (S3Exception $e) {
    echo "There was an error uploading the file.\n";
}