diff --git a/Models/NullProject.php b/Models/NullProject.php
index 4e18954..255cb28 100755
--- a/Models/NullProject.php
+++ b/Models/NullProject.php
@@ -36,4 +36,12 @@ final class NullProject extends Project
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProjectAttribute.php b/Models/NullProjectAttribute.php
index cfe5a6f..b16dbc0 100755
--- a/Models/NullProjectAttribute.php
+++ b/Models/NullProjectAttribute.php
@@ -36,4 +36,12 @@ final class NullProjectAttribute extends ProjectAttribute
         parent::__construct();
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProjectAttributeType.php b/Models/NullProjectAttributeType.php
index c71e59d..7b3bd3c 100755
--- a/Models/NullProjectAttributeType.php
+++ b/Models/NullProjectAttributeType.php
@@ -35,4 +35,12 @@ final class NullProjectAttributeType extends ProjectAttributeType
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProjectAttributeTypeL11n.php b/Models/NullProjectAttributeTypeL11n.php
index 30768bb..6147f6f 100755
--- a/Models/NullProjectAttributeTypeL11n.php
+++ b/Models/NullProjectAttributeTypeL11n.php
@@ -35,4 +35,12 @@ final class NullProjectAttributeTypeL11n extends ProjectAttributeTypeL11n
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }
diff --git a/Models/NullProjectAttributeValue.php b/Models/NullProjectAttributeValue.php
index aa88540..daee3db 100755
--- a/Models/NullProjectAttributeValue.php
+++ b/Models/NullProjectAttributeValue.php
@@ -35,4 +35,12 @@ final class NullProjectAttributeValue extends ProjectAttributeValue
     {
         $this->id = $id;
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function jsonSerialize() : mixed
+    {
+        return ['id' => $this->id];
+    }
 }