diff --git a/src/Exception.php b/src/Exception.php
index ce5df1d..a3704e7 100644
--- a/src/Exception.php
+++ b/src/Exception.php
@@ -51,7 +51,7 @@
  */
 class Exception extends \Exception
 {
-    public function __construct($message = "", $code = 0, \Throwable $previous = null)
+    public function __construct($message = "", $code = 0, ?\Throwable $previous = null)
     {
         if ($message == "") {
             $message = "libvips error: " . FFI::vips()->vips_error_buffer();