Unable to instantiate AWS SDK SNS MessageValidator\Message Class

I'm working through the excellent AWS SNS documentation here (http://ift.tt/PbyqyD) for handling incoming notifications, but am unable to get the following lines to work:

    try {
        // Create a message from the post data and validate its signature
        $message = Message::fromRawPostData();
        $validator = new MessageValidator();
        $validator->validate($message);
    } catch (\Exception $e) {
        // Pretend we're not here if the message is invalid
        throw new \Exception($e->getMessage());
    }

The error message I receive is:

Catchable Fatal Error: Argument 1 passed to Aws\Sns\MessageValidator\Message::fromArray() must be an array, null given

I'm using the correct vendor files and receive that error message with or without the 'vendor/autoload.php' file:

use Aws\Sns\MessageValidator\Message;
use Aws\Sns\MessageValidator\MessageValidator;

Other AWS SDK functions and objects work just fine, including all of the SnsClient functions documented here (http://ift.tt/1b3rA9C), which leads be to believe that the SDK is installed correctly.

Thanks in advance for any help!