diff --git a/CHANGELOG b/CHANGELOG
index 23808dd68..4076d0b3c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -2,6 +2,7 @@ CHANGELOG Roundcube Webmail
 ===========================
 
 - Managesieve: Fix locked UI after opening filter frame (#7007)
+- Fix PHP warning: "array_merge(): Expected parameter 2 to be an array, null given in sendmail.inc (#7003)
 
 RELEASE 1.4.0
 -------------
diff --git a/program/steps/mail/sendmail.inc b/program/steps/mail/sendmail.inc
index cfcaab6ea..19773656b 100644
--- a/program/steps/mail/sendmail.inc
+++ b/program/steps/mail/sendmail.inc
@@ -168,7 +168,7 @@ if ($files = rcube_utils::get_input_value('_attachments', rcube_utils::INPUT_POS
         unset($COMPOSE['attachments'][$idx]);
     }
 
-    $COMPOSE['attachments'] = array_merge(array_filter($files), $COMPOSE['attachments']);
+    $COMPOSE['attachments'] = array_merge(array_filter($files), (array) $COMPOSE['attachments']);
 }
 
 // Since we can handle big messages with disk usage, we need more time to work