diff --git a/src/FeedIo/Adapter/Guzzle/Response.php b/src/FeedIo/Adapter/Guzzle/Response.php
index d300e8c6..0f21915c 100644
--- a/src/FeedIo/Adapter/Guzzle/Response.php
+++ b/src/FeedIo/Adapter/Guzzle/Response.php
@@ -38,7 +38,7 @@ public function __construct(PsrResponseInterface $psrResponse)
      */
     public function isModified()
     {
-        return $this->psrResponse->getStatusCode() != 304 && strlen($this->psrResponse->getBody()) > 0;
+        return $this->psrResponse->getStatusCode() != 304 && $this->psrResponse->getBody()->getSize() > 0;
     }
 
     /**