diff --git a/Models/NullContact.php b/Models/NullContact.php
index e7886be..91babfe 100755
--- a/Models/NullContact.php
+++ b/Models/NullContact.php
@@ -36,4 +36,12 @@ final class NullContact extends Contact
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullContactElement.php b/Models/NullContactElement.php
index 1c4110d..2ef5c65 100755
--- a/Models/NullContactElement.php
+++ b/Models/NullContactElement.php
@@ -35,4 +35,12 @@ final class NullContactElement extends ContactElement
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProfile.php b/Models/NullProfile.php
index 90e805f..32e9b49 100755
--- a/Models/NullProfile.php
+++ b/Models/NullProfile.php
@@ -36,4 +36,12 @@ final class NullProfile extends Profile
         $this->id = $id;
         parent::__construct();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }