diff --git a/src/Database.php b/src/Database.php
index 71bc786..154ed23 100644
--- a/src/Database.php
+++ b/src/Database.php
@@ -10,6 +10,7 @@ final class Database
     {
         if (self::$pdo === null) {
             self::init();
+            assert(self::$pdo instanceof PDO);
             self::$pdo->exec((string) file_get_contents(dirname(__DIR__) . '/database/schema.sql'));
         }