diff --git a/lib/special_cases.php b/lib/special_cases.php
index 53244e45..2b368fce 100644
--- a/lib/special_cases.php
+++ b/lib/special_cases.php
@@ -204,7 +204,7 @@ function readdir($dir_handle = null)
  * @throws OpensslException
  *
  */
-function openssl_encrypt(string $data, string $method, string $key, int $options = 0, string $iv = "", string &$tag = "", string $aad = "", int $tag_length = 16): string
+function openssl_encrypt(string $data, string $method, string $key, int $options = 0, string $iv = "", ?string &$tag = "", string $aad = "", int $tag_length = 16): string
 {
     error_clear_last();
     // The $tag parameter is handled in a weird way by openssl_encrypt. It cannot be provided unless encoding is AEAD