Good point. Here’s the patch I’m running with that seems to work (now on 10.18.4)
--- class-s2-core.php 2015-02-10 00:44:36.000000000 +0000
+++ class-s2-core.php 2015-02-11 17:25:00.828988178 +0000
@@ -513,7 +513,7 @@
$plaintext = trim(strip_tags($plaintext));
- if ( strstr($mailtext, "{REFERENCELINKS}") && $plaintext_links != '' ) {
+ if ( isset( $plaintext_links ) && '' !== $plaintext_links ) {
$plaintext .= "\r\n\r\n" . trim($plaintext_links);
}