diff --git a/lib/classes/Swift/Transport/AbstractSmtpTransport.php b/lib/classes/Swift/Transport/AbstractSmtpTransport.php
index 39b6e2135..fa1edd533 100644
--- a/lib/classes/Swift/Transport/AbstractSmtpTransport.php
+++ b/lib/classes/Swift/Transport/AbstractSmtpTransport.php
@@ -443,6 +443,10 @@ protected function throwException(Swift_TransportException $e)
     /** Throws an Exception if a response code is incorrect */
     protected function assertResponseCode($response, $wanted)
     {
+        if (!$response) {
+            $this->throwException(new Swift_TransportException('Expected response code '.implode('/', $wanted).' but got an empty response'));
+        }
+
         list($code) = sscanf($response, '%3d');
         $valid = (empty($wanted) || in_array($code, $wanted));
 
@@ -452,12 +456,7 @@ protected function assertResponseCode($response, $wanted)
         }
 
         if (!$valid) {
-            $this->throwException(
-                new Swift_TransportException(
-                    'Expected response code '.implode('/', $wanted).' but got code '.
-                    '"'.$code.'", with message "'.$response.'"',
-                    $code)
-                );
+            $this->throwException(new Swift_TransportException('Expected response code '.implode('/', $wanted).' but got code "'.$code.'", with message "'.$response.'"', $code));
         }
     }