Seeing "debian/changelog not found" error when importing new upstream tarball

[deb251@lewenberg.com]

I am trying to build a Debian stretch package using an updated upstream 
tarball. However, I am getting the following error:

gbp.deb.changelog.NoChangeLogError: Changelog debian/changelog not found

Here is what I am doing.

1. Download the stretch source packages:

wget 
http://http.debian.net/debian/pool/main/h/heimdal/heimdal_7.1.0+dfsg-13+deb9u3.dsc
wget 
http://http.debian.net/debian/pool/main/h/heimdal/heimdal_7.1.0+dfsg.orig.tar.gz
wget 
http://http.debian.net/debian/pool/main/h/heimdal/heimdal_7.1.0+dfsg-13+deb9u3.debian.tar.xz

2. Import the source:

gbp import-dsc heimdal_7.1.0+dfsg-13+deb9u3.dsc

3. Download new source and change its name:

wget 
https://github.com/heimdal/heimdal/releases/download/heimdal-7.7.0/heimdal-7.7.0.tar.gz 

mv heimdal-7.7.0.tar.gz heimdal_7.7.0+dfsg.orig.tar.gz

4. Change into the directory created by the "gbp import-dsc " step:

cd heimdal

5. Import the upstream source:

git checkout upstream
gbp import-orig ../heimdal_7.7.0+dfsg.orig.tar.gz -u7.7.0

At this point I get this error:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gbp/scripts/import_orig.py", 
line 99, in detect_name_and_version
    cp = ChangeLog(filename='debian/changelog')
  File "/usr/lib/python3/dist-packages/gbp/deb/changelog.py", line 84, 
in __init__
    raise NoChangeLogError("Changelog %s not found" % (filename, ))
gbp.deb.changelog.NoChangeLogError: Changelog debian/changelog not found