diff --git a/Models/NullEmployee.php b/Models/NullEmployee.php
index 331505e..f56415a 100755
--- a/Models/NullEmployee.php
+++ b/Models/NullEmployee.php
@@ -36,4 +36,12 @@ final class NullEmployee extends Employee
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEmployeeEducationHistory.php b/Models/NullEmployeeEducationHistory.php
index bbc363a..61af77e 100755
--- a/Models/NullEmployeeEducationHistory.php
+++ b/Models/NullEmployeeEducationHistory.php
@@ -36,4 +36,12 @@ final class NullEmployeeEducationHistory extends EmployeeEducationHistory
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEmployeeHistory.php b/Models/NullEmployeeHistory.php
index a5911a3..551a7f5 100755
--- a/Models/NullEmployeeHistory.php
+++ b/Models/NullEmployeeHistory.php
@@ -36,4 +36,12 @@ final class NullEmployeeHistory extends EmployeeHistory
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullEmployeeWorkHistory.php b/Models/NullEmployeeWorkHistory.php
index 5698095..f25b862 100755
--- a/Models/NullEmployeeWorkHistory.php
+++ b/Models/NullEmployeeWorkHistory.php
@@ -36,4 +36,12 @@ final class NullEmployeeWorkHistory extends EmployeeWorkHistory
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }