diff --git a/tests/ConnectionTest.php b/tests/ConnectionTest.php
index f7926be5..fe853afd 100644
--- a/tests/ConnectionTest.php
+++ b/tests/ConnectionTest.php
@@ -117,7 +117,7 @@ static function (ConnectionInterface $db) {
         $db->close();
     }
 
-    public function testSerialized()
+    public function testSerialized(): void
     {
         $connection = $this->getConnection();
         $connection->open();