使用AWS PHP SDK导致PHP致命错误 - 调用未定义的方法Guzzle \\ Http \\ Message \\ Request :: batchWriteItem（）

I've a LAMP server on AWS EC2 and I've installed AWS SDK for PHP 2 using Composer, just like instructed here.

I'm trying to run a sample query to insert some items on DynamoDB, but I get the following error on PHP:

PHP Fatal error: Call to undefined method Guzzle\Http\Message\Request::batchWriteItem()

My code:

require "vendor/autoload.php";

use Aws\DynamoDb\DynamoDbClient;
use Aws\Common\Enum\Region;


// Instantiate the DynamoDB client with your AWS credentials
$aws = DynamoDbClient::factory(array(
  'key'    => 'xxx',
  'secret' => 'xxx',
  'region' => Region::US_WEST_2
));

$client = $aws->get("dynamodb");

$tableName = "xxxx";
$response = $client->batchWriteItem(array(
"RequestItems" => array(
    $tableName => array(
        array(
            "PutRequest" => array(
                "Item" => array(
                    "IP"            => array(Type::STRING => "203.203.203.22"),
                    "APIKEY" => array(Type::STRING => "jkhkjhskjhs"), 
                    "TIMESTAMP"       => array(Type::STRING => time()),
                    "PostedBy"      => array(Type::STRING => "User B")
                )
            )
        )
        ),
)
));

Ideas???

Thanks!