diff --git a/Util/ArrayConverter.php b/Util/ArrayConverter.php
index 635351e0..64e15b48 100644
--- a/Util/ArrayConverter.php
+++ b/Util/ArrayConverter.php
@@ -96,7 +96,10 @@ private static function cancelExpand(array &$tree, string $prefix, array $node):
         }
     }
 
-    private static function getKeyParts(string $key)
+    /**
+     * @return string[]
+     */
+    private static function getKeyParts(string $key): array
     {
         $parts = explode('.', $key);
         $partsCount = \count($parts);