filter/subjecttag: account for missing Subject: header

This is a high indicator of spam (but out-of-scope for this
particular module) but sometimes it is not, and people
legitimately forget to set a Subject: header at all.

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/PublicInbox/Filter/SubjectTag.pm b/lib/PublicInbox/Filter/SubjectTag.pm
index 1d281425..7735a94a 100644
--- a/lib/PublicInbox/Filter/SubjectTag.pm
+++ b/lib/PublicInbox/Filter/SubjectTag.pm
@@ -19,8 +19,10 @@ sub new {
 sub scrub {
         my ($self, $mime) = @_;
         my $subj = $mime->header('Subject');
-        $subj =~ s/$self->{tag_re}/$1/; # $1 is "Re: "
-        $mime->header_str_set('Subject', $subj);
+        if (defined $subj) {
+                $subj =~ s/$self->{tag_re}/$1/; # $1 is "Re: "
+                $mime->header_str_set('Subject', $subj);
+        }
         $self->ACCEPT($mime);
 }