diff --git a/src/OneSignalClient.php b/src/OneSignalClient.php
index 1a18adf..f6f92a7 100644
--- a/src/OneSignalClient.php
+++ b/src/OneSignalClient.php
@@ -147,7 +147,7 @@ public function sendNotificationToUser($message, $userId, $url = null, $data = n
         $params = array(
             'app_id' => $this->appId,
             'contents' => $contents,
-            'include_player_ids' => is_array($userId) ? $userId : array($userId)
+            'include_player_ids' => $this->transformUserId($userId)
         );
 
         if (isset($url)) {
@@ -171,7 +171,7 @@ public function sendNotificationToUser($message, $userId, $url = null, $data = n
                 "en" => $headings
             );
         }
-        
+
         if(isset($subtitle)){
             $params['subtitle'] = array(
                 "en" => $subtitle
@@ -199,7 +199,7 @@ public function sendNotificationToExternalUser($message, $userId, $url = null, $
         $params = array(
             'app_id' => $this->appId,
             'contents' => $contents,
-            'include_external_user_ids' => is_array($userId) ? $userId : array($userId)
+            'include_external_user_ids' => $this->transformUserId($userId)
         );
 
         if (isset($url)) {
@@ -264,7 +264,7 @@ public function sendNotificationUsingTags($message, $tags, $url = null, $data =
                 "en" => $headings
             );
         }
-        
+
         if(isset($subtitle)){
             $params['subtitle'] = array(
                 "en" => $subtitle
@@ -306,7 +306,7 @@ public function sendNotificationToAll($message, $url = null, $data = null, $butt
                 "en" => $headings
             );
         }
-        
+
         if(isset($subtitle)){
             $params['subtitle'] = array(
                 "en" => $subtitle
@@ -348,7 +348,7 @@ public function sendNotificationToSegment($message, $segment, $url = null, $data
                 "en" => $headings
             );
         }
-        
+
         if(isset($subtitle)){
             $params['subtitle'] = array(
                 "en" => $subtitle
@@ -415,7 +415,7 @@ public function getNotifications($app_id = null, $limit = null, $offset = null)
         $this->usesJSON();
 
         $endpoint = self::ENDPOINT_NOTIFICATIONS;
-        
+
         if(!$app_id) {
             $app_id = $this->appId;
         }
@@ -531,4 +531,19 @@ public function delete($endPoint) {
         }
         return $this->client->delete(self::API_URL . $endPoint, $this->headers);
     }
+
+    /**
+     * Transformed user_id as array string.
+     *
+     * @param  mixed  $userId
+     * @return array
+     */
+    private function transformUserId($userId)
+    {
+        $array = is_array($userId) ? $userId : array($userId);
+
+        return array_map(function ($item) {
+            return (string)$item;
+        }, $array);
+    }
 }