diff --git a/src/PackedBox.php b/src/PackedBox.php
index f0dbb28..3d5356f 100644
--- a/src/PackedBox.php
+++ b/src/PackedBox.php
@@ -236,7 +236,7 @@ public function jsonSerialize(): array
      * Validate that all items are placed solely within the confines of the box, and that no two items are placed
      * into the same physical space.
      */
-    private function assertPackingCompliesWithRealWorld(): bool
+    private function assertPackingCompliesWithRealWorld(): true
     {
         /** @var PackedItem[] $itemsToCheck */
         $itemsToCheck = iterator_to_array($this->items);