diff --git a/git-send-email.perl b/git-send-email.perl
index 4a2031084..3f50abaeb 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -538,7 +538,7 @@ sub send_message
 	send_message();
 
 	# set up for the next message
-	if ($chain_reply_to || length($reply_to) == 0) {
+	if ($chain_reply_to || !defined $reply_to || length($reply_to) == 0) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= " $message_id";