diff --git a/common/Alias/Model.php b/common/Alias/Model.php
index 06476af..9df62dd 100644
--- a/common/Alias/Model.php
+++ b/common/Alias/Model.php
@@ -6,9 +6,9 @@ use ProVM\Common\Factory\Model as ModelFactory;
 interface Model {
   public function getTable(): string;
   public function setFactory(ModelFactory $factory): Model;
-  public function parentOf(string $child_model_class, array $relation_definitions): array;
-  public function childOf(string $parent_model_class, array $relation_definitions): Model;
-  public function siblingOf(string $sibling_model_class, string $connecting_table, array $relation_definitions): array;
+  public function parentOf(string $child_model_class, array $relation_definitions): ?array;
+  public function childOf(string $parent_model_class, array $relation_definitions): ?Model;
+  public function siblingOf(string $sibling_model_class, string $connecting_table, array $relation_definitions): ?array;
 
   public function toArray(): array;
 }