diff --git a/lib/SimpleSAML/Session.php b/lib/SimpleSAML/Session.php
index 4d8360f4a5bd54624e6a3baa2a05570508c12a82..55851f9e7b90e758860efc8c5c7a632c7527e60d 100644
--- a/lib/SimpleSAML/Session.php
+++ b/lib/SimpleSAML/Session.php
@@ -222,7 +222,7 @@ class Session implements Serializable, Utils\ClearableState
      *
      * Cannot typehint param as string due to upstream restrictions
      */
-    public function unserialize($serialized)
+    public function unserialize($serialized): void
     {
         $session = unserialize($serialized);
         if (is_array($session)) {