diff --git a/src/Common/FacturaeSigner.php b/src/Common/FacturaeSigner.php
index b97ca04..9b69cde 100644
--- a/src/Common/FacturaeSigner.php
+++ b/src/Common/FacturaeSigner.php
@@ -176,7 +176,7 @@ public function sign($xml) {
     $certData = openssl_x509_parse($this->publicChain[0]);
     $certIssuer = [];
     foreach ($certData['issuer'] as $item=>$rawValues) {
-      if (!isset(self::ALLOWED_OID_TYPES[$item])) {
+      if (!array_key_exists($item, self::ALLOWED_OID_TYPES)) {
         continue;
       }
       $item = self::ALLOWED_OID_TYPES[$item];