diff --git a/src/Repository.php b/src/Repository.php
index abc8654..07da296 100644
--- a/src/Repository.php
+++ b/src/Repository.php
@@ -31,7 +31,7 @@ class Repository implements RepositoryInterface
 
     protected array $joined = [];
 
-    public function __construct(string $entity, string $alias = self::DEFAULT_ALIAS, ConnectionAdapter $adapter = null) {
+    public function __construct(string $entity, string $alias = self::DEFAULT_ALIAS, ?ConnectionAdapter $adapter = null) {
         $this->entityClass = $entity;
         $this->alias = $alias;
         $this->entityResolver = Ulmus::resolveEntity($entity);