diff --git a/lib/ResponseHandler.php b/lib/ResponseHandler.php
index 852b5f0..14c8a65 100644
--- a/lib/ResponseHandler.php
+++ b/lib/ResponseHandler.php
@@ -101,7 +101,7 @@ protected function makeJWS($payload, $exp_time, $iss, $aud, $jwk_pem): string {
             'data' => $payload,                                 // Authentication Data
         ];
 
-        if(array_key_exists('fiscalNumber', $payload)) {        // Subject - fiscalNumber, only if exists
+        if(is_array($payload) && array_key_exists('fiscalNumber', $payload)) {        // Subject - fiscalNumber, only if exists
             $data['sub'] = $payload['fiscalNumber'];
         }
 
@@ -119,4 +119,4 @@ protected function makeJWS($payload, $exp_time, $iss, $aud, $jwk_pem): string {
     
         return $token;
     }
-}
\ No newline at end of file
+}