diff --git a/Trustly/Api/api.php b/Trustly/Api/api.php
index c9fee7d..8cace65 100644
--- a/Trustly/Api/api.php
+++ b/Trustly/Api/api.php
@@ -171,7 +171,7 @@ public function serializeData($data) {
 	 *
 	 * @param string $uuid UUID in the API call
 	 *
-	 * @param string $signature in the API call
+	 * @param ?string $signature in the API call
 	 *
 	 * @param array $data in the API call
 	 *
@@ -484,9 +484,9 @@ public function call($request) {
 	/**
 	 * Return a boolean value formatted for communicating with the API.
 	 *
-	 * @param boolean $value Boolean value to encode
+	 * @param ?boolean $value Boolean value to encode
 	 *
-	 * @return API encoded boolean value
+	 * @return ?string API encoded boolean value
 	 */
 	protected function apiBool($value) {
 		if(isset($value)) {
diff --git a/Trustly/Data/data.php b/Trustly/Data/data.php
index a77274b..7320a35 100644
--- a/Trustly/Data/data.php
+++ b/Trustly/Data/data.php
@@ -54,9 +54,9 @@ public function __construct() {
 	 * values. This is used to keep the requests cleaner rather then
 	 * supplying NULL values in the payload
 	 *
-	 * @param array $data data to clean
+	 * @param mixed $data data to clean
 	 *
-	 * @return array cleaned data
+	 * @return mixed cleaned data
 	 */
 	public function vacuum($data) {
 		if(is_null($data)) {
@@ -106,7 +106,7 @@ public function get($name=NULL) {
 	 *
 	 * @param string $str String to process
 	 *
-	 * @return string UTF-8 variant of string
+	 * @return ?string UTF-8 variant of string
 	 */
 	public static function ensureUTF8($str) {
 		if($str == NULL) {
diff --git a/Trustly/Data/jsonrpcnotificationrequest.php b/Trustly/Data/jsonrpcnotificationrequest.php
index 6be3c5f..8bfc66a 100644
--- a/Trustly/Data/jsonrpcnotificationrequest.php
+++ b/Trustly/Data/jsonrpcnotificationrequest.php
@@ -135,7 +135,7 @@ public function getData($name=NULL) {
 	/**
 	 * Get the UUID from the request.
 	 *
-	 * @return string The UUID value
+	 * @return ?string The UUID value
 	 */
 	public function getUUID() {
 		return $this->getParams('uuid');
@@ -145,7 +145,7 @@ public function getUUID() {
 	/**
 	 * Get the Method from the request.
 	 *
-	 * @return string The Method value.
+	 * @return ?string The Method value.
 	 */
 	public function getMethod() {
 		return $this->get('method');
@@ -155,7 +155,7 @@ public function getMethod() {
 	/**
 	 * Get the Signature from the request.
 	 *
-	 * @return string The Signature value.
+	 * @return ?string The Signature value.
 	 */
 	public function getSignature() {
 		return $this->getParams('signature');
@@ -165,7 +165,7 @@ public function getSignature() {
 	/**
 	 * Get the JSON RPC version from the request.
 	 *
-	 * @return string The Version.
+	 * @return ?string The Version.
 	 */
 	public function getVersion() {
 		return $this->get('version');
diff --git a/Trustly/Data/jsonrpcnotificationresponse.php b/Trustly/Data/jsonrpcnotificationresponse.php
index 87894a8..33b4d1d 100644
--- a/Trustly/Data/jsonrpcnotificationresponse.php
+++ b/Trustly/Data/jsonrpcnotificationresponse.php
@@ -73,7 +73,7 @@ public function __construct($request, $success=NULL) {
 	 * @param boolean $success Set to true to indicate that the notification
 	 *		was successfully processed.
 	 *
-	 * @return $success
+	 * @return ?boolean $success
 	 */
 	public function setSuccess($success=NULL) {
 		$status = 'OK';
@@ -198,7 +198,7 @@ public function setData($name, $value) {
 	/**
 	 * Get the Method value from the response.
 	 *
-	 * @return string The Method value.
+	 * @return ?string The Method value.
 	 */
 	public function getMethod() {
 		return $this->getResult('method');
@@ -208,7 +208,7 @@ public function getMethod() {
 	/**
 	 * Get the UUID value from the response.
 	 *
-	 * @return string The UUID value
+	 * @return ?string The UUID value
 	 */
 	public function getUUID() {
 		return $this->getResult('uuid');
diff --git a/Trustly/Data/jsonrpcrequest.php b/Trustly/Data/jsonrpcrequest.php
index 2b10f9a..35bb820 100644
--- a/Trustly/Data/jsonrpcrequest.php
+++ b/Trustly/Data/jsonrpcrequest.php
@@ -149,7 +149,7 @@ public function setUUID($uuid) {
 	/**
 	 * Get the UUID value from the outgoing call.
 	 *
-	 * @return string The UUID value
+	 * @return ?string The UUID value
 	 */
 	public function getUUID() {
 		if(isset($this->payload['params']['UUID'])) {
@@ -173,7 +173,7 @@ public function setMethod($method) {
 	/**
 	 * Get the Method value from the outgoing call.
 	 *
-	 * @return string The Method value.
+	 * @return ?string The Method value.
 	 */
 	public function getMethod() {
 		return $this->get('method');
diff --git a/Trustly/Data/jsonrpcresponse.php b/Trustly/Data/jsonrpcresponse.php
index 98a2c4c..a53a72e 100644
--- a/Trustly/Data/jsonrpcresponse.php
+++ b/Trustly/Data/jsonrpcresponse.php
@@ -78,7 +78,7 @@ public function __construct($response_body) {
 	/**
 	 * Get error code (if any) from the API response
 	 *
-	 * @return integer The error code (numerical)
+	 * @return ?integer The error code (numerical)
 	 */
 	public function getErrorCode() {
 		if($this->isError() && isset($this->response_result['code'])) {
@@ -90,7 +90,7 @@ public function getErrorCode() {
 	/**
 	 * Get error message (if any) from the API response
 	 *
-	 * @return string The error message
+	 * @return ?string The error message
 	 */
 	public function getErrorMessage() {
 		if($this->isError() && isset($this->response_result['message'])) {
diff --git a/Trustly/Data/jsonrpcsignedresponse.php b/Trustly/Data/jsonrpcsignedresponse.php
index 4b16b02..e26fa7a 100644
--- a/Trustly/Data/jsonrpcsignedresponse.php
+++ b/Trustly/Data/jsonrpcsignedresponse.php
@@ -116,7 +116,7 @@ public function getData($name=NULL) {
 	/**
 	 * Get error code (if any) from the API call
 	 *
-	 * @return integer The error code (numerical)
+	 * @return ?integer The error code (numerical)
 	 */
 	public function getErrorCode() {
 		if($this->isError() && isset($this->response_result['data']['code'])) {
@@ -129,7 +129,7 @@ public function getErrorCode() {
 	/**
 	 * Get error message (if any) from the API call
 	 *
-	 * @return string The error message
+	 * @return ?string The error message
 	 */
 	public function getErrorMessage() {
 		if($this->isError() && isset($this->response_result['data']['message'])) {
diff --git a/Trustly/Data/request.php b/Trustly/Data/request.php
index 3daaca7..6a0b776 100644
--- a/Trustly/Data/request.php
+++ b/Trustly/Data/request.php
@@ -36,7 +36,7 @@
 class Trustly_Data_Request extends Trustly_Data {
 	/**
 	 * Call method name
-	 * @var string
+	 * @var ?string
 	 */
 	var $method = NULL;
 
@@ -62,7 +62,7 @@ public function __construct($method=NULL, $payload=NULL) {
 	/**
 	 * Convenience function for getting the uuid from the call
 	 *
-	 * @return string uuid
+	 * @return ?string uuid
 	 */
 	public function getUUID() {
 		if(isset($this->payload['uuid'])) {
@@ -85,7 +85,7 @@ public function setUUID($uuid) {
 	/**
 	 * Get the method in the outgoing call
 	 *
-	 * @return string method name
+	 * @return ?string method name
 	 */
 	public function getMethod() {
 		return $this->method;
diff --git a/Trustly/Data/response.php b/Trustly/Data/response.php
index 0f4b97b..4c6e682 100644
--- a/Trustly/Data/response.php
+++ b/Trustly/Data/response.php
@@ -135,7 +135,7 @@ public function isSuccess() {
 	/**
 	 * Get error message (if any) from the API response
 	 *
-	 * @return string The error message
+	 * @return ?string The error message
 	 */
 	public function getErrorMessage() {
 		if($this->isError()) {
@@ -150,7 +150,7 @@ public function getErrorMessage() {
 	/**
 	 * Get error code (if any) from the API response
 	 *
-	 * @return integer The error code (numerical)
+	 * @return ?integer The error code (numerical)
 	 */
 	public function getErrorCode() {
 		if($this->isError()) {
@@ -188,7 +188,7 @@ public function getResult($name=NULL) {
 	/**
 	 * Convenience function for getting the uuid in the response
 	 *
-	 * @param string uuid
+	 * @return ?string uuid
 	 */
 	public function getUUID() {
 		if(isset($this->response_result['uuid'])) {
@@ -201,7 +201,7 @@ public function getUUID() {
 	/**
 	 * Get the method from the response
 	 *
-	 * @return string method name
+	 * @return ?string method name
 	 */
 	public function getMethod() {
 		if(isset($this->response_result['method'])) {
@@ -214,7 +214,7 @@ public function getMethod() {
 	/**
 	 * Get the signature from the response
 	 *
-	 * @return string signature
+	 * @return ?string signature
 	 */
 	public function getSignature() {
 		if(isset($this->response_result['signature'])) {