taper ignores the long option --unattended-file

I was trying to backup a file set with taper in unattended mode using the
following command line:

/usr/sbin/taper \
  --incremental-backup-on \
  --archive-title "Nightly backup; tape inserted $TODAY" \
  --volume-title "Backup done $TODAY" \
  --unattended-file '@nightly'

This produced an empty backup.  That was confusing, because running the
same file set in interactive mode worked well.

Later, I tried replacing "--unattended-file @nightly" with the shorthand
notion "-U @nightly" and that worked fine.