diff --git a/phpBB/includes/functions_privmsgs.php b/phpBB/includes/functions_privmsgs.php
index 0aceeb90e1..4e3fd5a85e 100644
--- a/phpBB/includes/functions_privmsgs.php
+++ b/phpBB/includes/functions_privmsgs.php
@@ -1689,7 +1689,7 @@ function submit_pm($mode, $subject, &$data_ary, $put_in_outbox = true)
 	}
 
 	// First of all make sure the subject are having the correct length.
-	$subject = truncate_string($subject);
+	$subject = truncate_string($subject, $mode === 'post' ? 120 : 124);
 
 	$db->sql_transaction('begin');