diff --git a/Entity/DeviceNotification.php b/Entity/DeviceNotification.php
index 567c6ea..97776a2 100644
--- a/Entity/DeviceNotification.php
+++ b/Entity/DeviceNotification.php
@@ -38,6 +38,10 @@ public function setDeviceToken($token)
      */
     public function addDeviceToken($token)
     {
+        if(is_string($token) === false){
+            throw new \InvalidArgumentException('Token must be a string!');
+        }
+
         $this->deviceTokens->add($token);
 
         return $this;
@@ -48,6 +52,10 @@ public function addDeviceToken($token)
      */
     public function setDeviceTokens($tokens)
     {
+        if(is_array($tokens) === false){
+            throw new \InvalidArgumentException('Tokens must be an array of strings!');
+        }
+
         $this->deviceTokens = new ArrayCollection($tokens);
 
         return $this;
diff --git a/FCMClient.php b/FCMClient.php
index 3fae27a..54e30d8 100644
--- a/FCMClient.php
+++ b/FCMClient.php
@@ -48,7 +48,7 @@ public function __construct(Client $client)
      * @param null | array $token
      * @return DeviceNotification
      */
-    public function createDeviceNotification($title = null, $body = null, $token = null)
+    public function createDeviceNotification($title = null, $body = null, $token = array())
     {
         $notification = new DeviceNotification();
         $notification