diff --git a/src/Fetch/Attachment.php b/src/Fetch/Attachment.php
index e77984a..c9db5e3 100644
--- a/src/Fetch/Attachment.php
+++ b/src/Fetch/Attachment.php
@@ -98,7 +98,9 @@ public function __construct(Message $message, $structure, $partIdentifier = null
             $this->setFileName($parameters['name']);
         }
 
-        $this->size = $structure->bytes;
+        if (isset($structure->bytes)) {
+            $this->size = $structure->bytes;
+        }
 
         $this->mimeType = Message::typeIdToString($structure->type);