diff --git a/api/v3/Mailing.php b/api/v3/Mailing.php
index b0e868490cff..c9819ba00204 100644
--- a/api/v3/Mailing.php
+++ b/api/v3/Mailing.php
@@ -584,7 +584,7 @@ function civicrm_api3_mailing_preview($params) {
   return civicrm_api3_create_success(array(
     'id' => $params['id'],
     'contact_id' => $contactID,
-    'subject' => $mime->_headers['Subject'],
+    'subject' => $mime->headers()['Subject'],
     'body_html' => $mime->getHTMLBody(),
     'body_text' => $mime->getTXTBody(),
   ));