diff --git a/src/Sylius/Bundle/AdminBundle/TwigComponent/Promotion/FormComponent.php b/src/Sylius/Bundle/AdminBundle/TwigComponent/Promotion/FormComponent.php
index 812c7e0da9b..3facda289a0 100644
--- a/src/Sylius/Bundle/AdminBundle/TwigComponent/Promotion/FormComponent.php
+++ b/src/Sylius/Bundle/AdminBundle/TwigComponent/Promotion/FormComponent.php
@@ -59,11 +59,9 @@ public function addCollectionItem(PropertyAccessorInterface $propertyAccessor, #
             $data = [];
         }
 
-        $index = [] !== $data ? max(array_keys($data)) + 1 : 0;
-
         $propertyAccessor->setValue(
             $this->formValues,
-            $propertyPath."[$index]",
+            sprintf('%s[%d]', $propertyPath, $this->resolveItemIndex($data)),
             ['type' => $this->provideItemType($name)],
         );
     }
@@ -85,4 +83,10 @@ private function provideItemType(string $name): string
 
         return '';
     }
+
+    /** @param array<array-key, array<string, mixed>> $data */
+    private function resolveItemIndex(array $data): int
+    {
+        return [] !== $data ? max(array_keys($data)) + 1 : 0;
+    }
 }