Comment 0 for bug 1885233

update-maintainer is fussy about only being run in the package's root:

    ~/pkg/Nginx/merge-v1.18.0-3/nginx-gu/debian$ update-maintainer
    No control file found in ./debian.

The -d option provides a way to work around this, but is a tad awkward:

    ~/pkg/Nginx/merge-v1.18.0-3/nginx-gu/debian$ update-maintainer -d../debian
    The Maintainer email is set to an ubuntu.com address. Doing nothing.

Packagers often work from in the debian/ dir itself so it would be convenient to scan up the directory tree (perhaps a few levels?)