diff --git a/common/Define/Model.php b/common/Define/Model.php
index 5b2a59e..5be2247 100644
--- a/common/Define/Model.php
+++ b/common/Define/Model.php
@@ -37,7 +37,7 @@ abstract class Model extends BaseModel implements ModelInterface {
     }
     return $definitions;
   }
-  public function parentOf(string $child_model_class, array $relation_definitions): array {
+  public function parentOf(string $child_model_class, array $relation_definitions): ?array {
     $relation_definitions = $this->checkDefinitions($relation_definitions, [
       Model::SELF_KEY,
       Model::CHILD_KEY
@@ -54,7 +54,7 @@ abstract class Model extends BaseModel implements ModelInterface {
       ])
       ->many();
   }
-  public function childOf(string $parent_model_class, array $relation_definitions): ModelInterface {
+  public function childOf(string $parent_model_class, array $relation_definitions): ?ModelInterface {
     $relation_definitions = $this->checkDefinitions($relation_definitions, [
       Model::SELF_KEY,
       Model::PARENT_KEY
@@ -72,7 +72,7 @@ abstract class Model extends BaseModel implements ModelInterface {
       ])
       ->one();
   }
-  public function siblingOf(string $sibling_model_class, string $connecting_table, array $relation_definitions): array {
+  public function siblingOf(string $sibling_model_class, string $connecting_table, array $relation_definitions): ?array {
     $relation_definitions = $this->checkDefinitions($relation_definitions, [
       Model::SELF_KEY,
       Model::SIBLING_KEY,