lib/PublicInbox/Hval.pm   | 4 ++--
 lib/PublicInbox/Mbox.pm   | 4 ++--
 lib/PublicInbox/MboxGz.pm | 3 +--
 t/hval.t                  | 8 +++++---

diff --git a/lib/PublicInbox/Hval.pm b/lib/PublicInbox/Hval.pm
index 46a839160c3f996e25eb4e0b91b4a1a82fdfabef..e21a64a60e6690636555c2d7dc52473301ba5d25 100644
--- a/lib/PublicInbox/Hval.pm
+++ b/lib/PublicInbox/Hval.pm
@@ -94,12 +94,12 @@ }
 
 # like format_sanitized_subject in git.git pretty.c with '%f' format string
 sub to_filename ($) {
-	my ($s, undef) = split(/\n/, $_[0]);
+	my $s = (split(/\n/, $_[0]))[0] // return; # empty string => undef
 	$s =~ s/[^A-Za-z0-9_\.]+/-/g;
 	$s =~ tr/././s;
 	$s =~ s/[\.\-]+\z//;
 	$s =~ s/\A[\.\-]+//;
-	$s
+	$s eq '' ? undef : $s;
 }
 
 # convert a filename (or any string) to HTML attribute
diff --git a/lib/PublicInbox/Mbox.pm b/lib/PublicInbox/Mbox.pm
index 8726b9f641b248e9f581f568a4eba67fb91e3657..115321c610f70be296f46dcf1b2de38512fcd006 100644
--- a/lib/PublicInbox/Mbox.pm
+++ b/lib/PublicInbox/Mbox.pm
@@ -52,9 +52,9 @@ }
 
 sub res_hdr ($$) {
 	my ($ctx, $subject) = @_;
-	my $fn = $subject // 'no-subject';
+	my $fn = $subject // '';
 	$fn =~ s/^re:\s+//i;
-	$fn = $fn eq '' ? 'no-subject' : to_filename($fn);
+	$fn = to_filename($fn) // 'no-subject';
 	my @hdr = ('Content-Type');
 	if ($ctx->{-inbox}->{obfuscate}) {
 		# obfuscation is stupid, but maybe scrapers are, too...
diff --git a/lib/PublicInbox/MboxGz.pm b/lib/PublicInbox/MboxGz.pm
index fdd16f68e35f91c44131b65de786f8f1037a2a92..967af9c68ad1bf9e4961fbb86aa4ddc1bd2ddee4 100644
--- a/lib/PublicInbox/MboxGz.pm
+++ b/lib/PublicInbox/MboxGz.pm
@@ -24,8 +24,7 @@ 	my ($self, $cb, $fn) = @_;
 	$self->{cb} = $cb;
 	$self->{base_url} = $self->{-inbox}->base_url($self->{env});
 	$self->{gz} = PublicInbox::GzipFilter::gzip_or_die();
-	$fn = to_filename($fn // 'no-subject');
-	$fn = 'no-subject' if $fn eq '';
+	$fn = to_filename($fn // '') // 'no-subject';
 	# http://www.iana.org/assignments/media-types/application/gzip
 	bless $self, __PACKAGE__;
 	my $res_hdr = [ 'Content-Type' => 'application/gzip',
diff --git a/t/hval.t b/t/hval.t
index 38605c6f16faf6e4b424af66a7306446a27334c7..e80a02ff40879657678425852c7c9f26c541df32 100644
--- a/t/hval.t
+++ b/t/hval.t
@@ -47,14 +47,16 @@ EOF
 
 is($html, $exp, 'only obfuscated relevant addresses');
 
-is('foo-bar', PublicInbox::Hval::to_filename('foo bar  '),
+is(PublicInbox::Hval::to_filename('foo bar  '), 'foo-bar',
 	'to_filename has no trailing -');
 
-is('foo-bar', PublicInbox::Hval::to_filename("foo   bar\nanother line\n"),
+is(PublicInbox::Hval::to_filename("foo   bar\nanother line\n"), 'foo-bar',
 	'to_filename has no repeated -, and nothing past LF');
 
-is('foo.bar', PublicInbox::Hval::to_filename("foo....bar"),
+is(PublicInbox::Hval::to_filename("foo....bar"), 'foo.bar',
 	'to_filename squeezes -');
+
+is(PublicInbox::Hval::to_filename(''), undef, 'empty string returns undef');
 
 my $s = "\0\x07\n";
 PublicInbox::Hval::src_escape($s);