diff --git a/source/CAS.php b/source/CAS.php
index df6bc825..da145bc1 100644
--- a/source/CAS.php
+++ b/source/CAS.php
@@ -347,7 +347,7 @@ class phpCAS
      */
     public static function client($server_version, $server_hostname,
         $server_port, $server_uri, $service_base_url,
-        $changeSessionID = true, \SessionHandlerInterface $sessionHandler = null
+        $changeSessionID = true, ?\SessionHandlerInterface $sessionHandler = null
     ) {
         phpCAS :: traceBegin();
         if (is_object(self::$_PHPCAS_CLIENT)) {
@@ -402,7 +402,7 @@ public static function client($server_version, $server_hostname,
      */
     public static function proxy($server_version, $server_hostname,
         $server_port, $server_uri, $service_base_url,
-        $changeSessionID = true, \SessionHandlerInterface $sessionHandler = null
+        $changeSessionID = true, ?\SessionHandlerInterface $sessionHandler = null
     ) {
         phpCAS :: traceBegin();
         if (is_object(self::$_PHPCAS_CLIENT)) {
diff --git a/source/CAS/Client.php b/source/CAS/Client.php
index 8ca9711f..42709e91 100644
--- a/source/CAS/Client.php
+++ b/source/CAS/Client.php
@@ -938,7 +938,7 @@ public function __construct(
         $server_uri,
         $service_base_url,
         $changeSessionID = true,
-        \SessionHandlerInterface $sessionHandler = null
+        ?\SessionHandlerInterface $sessionHandler = null
     ) {
         // Argument validation
         if (gettype($server_version) != 'string')