Hi,

When a message does not contain a subject, the "Control:" lines are not
processed but cause an error; see attached mail.

Best regards

Ole
Processing control commands:
the allowed types: scalar
.

I can confirm that this bug is still around. Mesage #10 of bug 989430
[0], which had no subject line, resulted in

    Failed to alter tags of Bug 989430: The 'request_subject' parameter
    (undef) to Debbugs::Control::set_tag was an 'undef', which is not
    one of the allowed types: scalar

Best,
Ryan

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=989430#10