diff --git a/backend/app/Element/ElementRepository.php b/backend/app/Element/ElementRepository.php
index b9ad71f..86631bd 100644
--- a/backend/app/Element/ElementRepository.php
+++ b/backend/app/Element/ElementRepository.php
@@ -2,8 +2,13 @@
 
 namespace App\Element;
 
+use DomainException;
+
 interface ElementRepository
 {
+    /**
+     * @throws DomainException
+     */
     public function create(CreateElementDto $dto): Element;
 
     public function find(int $id): ?Element;
diff --git a/backend/app/Element/EloquentElementRepository.php b/backend/app/Element/EloquentElementRepository.php
index a1a2709..3b8bc7b 100644
--- a/backend/app/Element/EloquentElementRepository.php
+++ b/backend/app/Element/EloquentElementRepository.php
@@ -4,6 +4,7 @@ namespace App\Element;
 
 use App\Set\Set;
 use App\Set\SetRepository;
+use DomainException;
 use RuntimeException;
 
 class EloquentElementRepository implements ElementRepository
@@ -14,6 +15,7 @@ class EloquentElementRepository implements ElementRepository
 
     public function create(CreateElementDto $dto): Element
     {
+        $this->validateParentSet($dto);
         $position = $this->nextPosition(
             $dto->set,
             $dto->parentElement,
@@ -43,6 +45,23 @@ class EloquentElementRepository implements ElementRepository
         return $model === null ? null : $this->toDomain($model);
     }
 
+    /**
+     * @throws DomainException
+     */
+    private function validateParentSet(CreateElementDto $dto): void
+    {
+        $parentElement = $dto->parentElement;
+        if ($parentElement === null) {
+            return;
+        }
+
+        if ($parentElement->getSet()->getId() !== $dto->set->getId()) {
+            throw new DomainException(
+                'parent element must belong to the same set',
+            );
+        }
+    }
+
     private function nextPosition(
         Set $set,
         ?Element $parentElement,
diff --git a/backend/tests/Fakes/FakeElementRepository.php b/backend/tests/Fakes/FakeElementRepository.php
index 4746bf3..ebe197d 100644
--- a/backend/tests/Fakes/FakeElementRepository.php
+++ b/backend/tests/Fakes/FakeElementRepository.php
@@ -5,6 +5,7 @@ namespace Tests\Fakes;
 use App\Element\CreateElementDto;
 use App\Element\Element;
 use App\Element\ElementRepository;
+use DomainException;
 
 class FakeElementRepository implements ElementRepository
 {
@@ -15,6 +16,7 @@ class FakeElementRepository implements ElementRepository
 
     public function create(CreateElementDto $dto): Element
     {
+        $this->validateParentSet($dto);
         $id = count($this->elements) + 1;
         $element = new Element(
             id: $id,
@@ -36,6 +38,23 @@ class FakeElementRepository implements ElementRepository
         return $element === null ? null : $this->copy($element);
     }
 
+    /**
+     * @throws DomainException
+     */
+    private function validateParentSet(CreateElementDto $dto): void
+    {
+        $parentElement = $dto->parentElement;
+        if ($parentElement === null) {
+            return;
+        }
+
+        if ($parentElement->getSet()->getId() !== $dto->set->getId()) {
+            throw new DomainException(
+                'parent element must belong to the same set',
+            );
+        }
+    }
+
     private function nextPosition(CreateElementDto $dto): int
     {
         $requestedParentId = $dto->parentElement?->getId();