diff --git a/Models/NullDepartment.php b/Models/NullDepartment.php
index 159c7c7..c795543 100755
--- a/Models/NullDepartment.php
+++ b/Models/NullDepartment.php
@@ -36,4 +36,12 @@ final class NullDepartment extends Department
         $this->id   = $id;
         $this->unit = new NullUnit();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullPosition.php b/Models/NullPosition.php
index 7fe8826..2cd12dc 100755
--- a/Models/NullPosition.php
+++ b/Models/NullPosition.php
@@ -35,4 +35,12 @@ final class NullPosition extends Position
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullUnit.php b/Models/NullUnit.php
index 60d59ab..c164afd 100755
--- a/Models/NullUnit.php
+++ b/Models/NullUnit.php
@@ -38,4 +38,12 @@ final class NullUnit extends Unit
         $this->id    = $id;
         $this->image = new NullMedia();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }