lib/PublicInbox.pm | 1 +

diff --git a/lib/PublicInbox.pm b/lib/PublicInbox.pm
index a4aba34692852263c9d5abe8e927673f958a1444..b05fd8c5607853af1050054162818b6ecbdd4706 100644
--- a/lib/PublicInbox.pm
+++ b/lib/PublicInbox.pm
@@ -19,6 +19,7 @@ 	my ($klass, $filter, $recipient) = @_;
 	my $simple = $filter->simple;
 	return 0 unless $simple->header("Message-ID");
 	return 0 unless defined($filter->from);
+	return 0 unless $simple->header("Subject");
 	return 0 if length($simple->as_string) > MAX_SIZE;
 	recipient_specified($filter, $recipient);
 }