AWS PHP SDK ~ Why am I getting this error "The command must be prepared before retrieving the request"

I am using the latest PHP AWS SDK... I seem to be running into this problem in certain circumstances... for example:

require 'aws-autoloader.php';
use Aws\S3\S3Client;

$client = S3Client::factory(array(
   'key'    => <skey>,
   'secret' => <secret>
));

$key = $_GET['key'];

try {

$client -> copyObject(array(
  'ACL' => "private",
  'Bucket' => "<bucket>",
  'Key' => "{$key}-copy",
  'CopySource' => "<bucket>/{$key}",
  'ContentType'     => "text/html",   
  'CacheControl' => "max-age=1, private",
  'MetadataDirective' => "REPLACE"
));

} catch (Exception $e) {
echo json_encode($e->getMessage());
}  

?>

Why am I getting this error, "The command must be prepared before retrieving the request?"