aws sdk2 s3 service results in exception

appDevDebugProjectContainer

    protected function getAwsS3Service()
    {
        return $this->services['aws_s3'] = call_user_func(array('Aws\\S3\\S3Client', 'factory'), 'secret', 'key');
    }

controller.php

$this->get('aws_s3');
//results in
//Warning: Illegal string offset 'client.backoff' in ../vendor/aws/aws-sdk-php/src/Aws/S3/S3Client.php line 186 

services.yml

aws_s3:
    class: Aws\S3\S3Client
    factory_class: Aws\S3\S3Client
    factory_method: factory
    arguments: [%aws_key%, %aws_secret%]