diff --git a/tests/AbstractConnectionTest.php b/tests/AbstractConnectionTest.php
index 311841f2b..5c214e04a 100644
--- a/tests/AbstractConnectionTest.php
+++ b/tests/AbstractConnectionTest.php
@@ -139,7 +139,7 @@ public function testSetTablePrefix(): void
         $this->assertSame('pre_', $db->getTablePrefix());
     }
 
-    public function testSerialized()
+    public function testSerialized(): void
     {
         $connection = $this->getConnection();
         $connection->open();