diff --git a/src/Conversation.php b/src/Conversation.php
index 299392d..c62480d 100644
--- a/src/Conversation.php
+++ b/src/Conversation.php
@@ -225,7 +225,7 @@ public function getAcls()
      *
      * @return self
      */
-    public function addMessage($message, $type = "message", $metas = null)
+    public function addMessage($message, $type = "message", array $metas = null)
     {
         if (false === is_string($message)) {
             throw new \Exception("Message given to addMessage method needs to be of type string");