diff --git a/src/OneSignalClient.php b/src/OneSignalClient.php
index a37d126..8512094 100644
--- a/src/OneSignalClient.php
+++ b/src/OneSignalClient.php
@@ -218,7 +218,7 @@ public function sendNotificationCustom($parameters = []){
         $this->requiresAuth();
         $this->usesJSON();
         // By Sucipto
-        if ($parameters['api_key']) {
+        if (isset($parameters['api_key'])) {
             $this->headers['headers']['Authorization'] = 'Basic '.$parameters['api_key'];
         }
         // Make sure to use app_id