public function subscribe($topicArn, $protocol, $endpoint) {
		if(empty($topicArn) || empty($protocol) || empty($endpoint)) {
			throw new InvalidArgumentException('Must supply a TopicARN, Protocol and Endpoint to subscribe to a topic');
		}

		$response = $this->_request('Subscribe', array(
			'TopicArn' => $topicArn,
			'Protocol' => $protocol,
			'Endpoint' => $endpoint
		));

		return strval($response->SubscribeResult->SubscriptionArn);
	}