diff --git a/src/Form/DataTransformer/RoomSelectionToIdTransformer.php b/src/Form/DataTransformer/RoomSelectionToIdTransformer.php
index 0a30674..d1775fc 100644
--- a/src/Form/DataTransformer/RoomSelectionToIdTransformer.php
+++ b/src/Form/DataTransformer/RoomSelectionToIdTransformer.php
@@ -59,12 +59,8 @@ class RoomSelectionToIdTransformer implements DataTransformerInterface
             return null;
         }
 
-        if (is_int($value)) {
-            return $value;
-        }
-
-        if (is_string($value) && ctype_digit($value)) {
-            return (int) $value;
+        if ($value instanceof RoomSelectionDto) {
+            return $value->id;
         }
 
         throw new TransformationFailedException(sprintf(
diff --git a/tests/Form/DataTransformer/RoomSelectionToIdTransformerTest.php b/tests/Form/DataTransformer/RoomSelectionToIdTransformerTest.php
index 16bccec..f4e6496 100644
--- a/tests/Form/DataTransformer/RoomSelectionToIdTransformerTest.php
+++ b/tests/Form/DataTransformer/RoomSelectionToIdTransformerTest.php
@@ -11,12 +11,15 @@ use Symfony\Component\Form\Exception\TransformationFailedException;
 
 class RoomSelectionToIdTransformerTest extends TestCase
 {
-    public function testReverseTransformAcceptsIntegerAndNumericStringIds(): void
+    public function testReverseTransformAcceptsRoomSelectionDto(): void
     {
-        $transformer = new RoomSelectionToIdTransformer([]);
+        $roomSelection = new RoomSelectionDto();
+        $roomSelection->id = 7;
+        $roomSelection->label = 'Doppelzimmer';
 
-        self::assertSame(12, $transformer->reverseTransform(12));
-        self::assertSame(12, $transformer->reverseTransform('12'));
+        $transformer = new RoomSelectionToIdTransformer([$roomSelection]);
+
+        self::assertSame(7, $transformer->reverseTransform($roomSelection));
     }
 
     public function testReverseTransformReturnsNullForEmptyValues(): void
@@ -33,7 +36,7 @@ class RoomSelectionToIdTransformerTest extends TestCase
 
         $this->expectException(TransformationFailedException::class);
 
-        $transformer->reverseTransform('1.5');
+        $transformer->reverseTransform('12');
     }
 
     public function testTransformReturnsMatchingRoomSelectionDto(): void