diff --git a/src/Fetch/Attachment.php b/src/Fetch/Attachment.php
index e77984a..4079c5a 100644
--- a/src/Fetch/Attachment.php
+++ b/src/Fetch/Attachment.php
@@ -97,8 +97,10 @@ public function __construct(Message $message, $structure, $partIdentifier = null
         } elseif (isset($parameters['name'])) {
             $this->setFileName($parameters['name']);
         }
-
-        $this->size = $structure->bytes;
+        
+        if (isset($structure->bytes)) {
+            $this->size = $structure->bytes;
+        }
 
         $this->mimeType = Message::typeIdToString($structure->type);