diff --git a/src/Feed.php b/src/Feed.php
index 0115546..84e3f95 100644
--- a/src/Feed.php
+++ b/src/Feed.php
@@ -91,6 +91,8 @@ protected function lastUpdated(): string
             return '';
         }
 
-        return $this->items->sortBy('updated')->last()->updated->toAtomString();
+        return $this->items->sortBy(function ($feedItem) {
+            return $feedItem->updated;
+        })->last()->updated->toAtomString();
     }
 }