diff --git a/src/MandrillApiTransport.php b/src/MandrillApiTransport.php
index 3f487bb..6ab4170 100644
--- a/src/MandrillApiTransport.php
+++ b/src/MandrillApiTransport.php
@@ -251,6 +251,9 @@ protected function logMessageContent(Email $message, $results = [])
         $contentType = $message->getHtmlBody() !== null ? "text/html" : "text";
 
         $logContent = $body;
+        if (is_object($logContent)) {
+            $logContent = $logContent->toString();
+        }
 
         // Append some extra information at the end
         $logContent .= '<hr><pre>Debug infos:' . "\n\n";