Doesn't work on first run, fails to create archive

[xenithorb]

(dgroc) [dgroc@icarus ~]$ python dgroc/dgroc.py --debug
Processing project: rocketchat-desktop
Generating new source rpm for project: rocketchat-desktop
Cloning https://github.com/RocketChat/Rocket.Chat.Electron.git
Last commit: ac8d04ec
Command to generate archive: git archive --format=tar --prefix=rocketchat-desktop/ -o/home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar HEAD
Update spec file: /home/dgroc/fedora-specs/copr/rocketchat-dev/rocketchat.spec
Release line before: Release:   102.git%{_shortcommit}%{?dist}
Release number: 103
Release line after: Release:        103.20170506gitac8d04ec%{?dist}
Source0 line after: Source0:        rocketchat-desktop-ac8d04ec.tar
Spec file updated: /home/dgroc/fedora-specs/copr/rocketchat-dev/rocketchat.spec
Strange result of the rpmbuild -bs:
  stdout:
  stderr:error: File /home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar: No such file or directory

0 srpms generated
(dgroc) [dgroc@icarus ~]$ ls -l /home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar
ls: cannot access '/home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar': No such file or directory
(dgroc) [dgroc@icarus ~]$
(dgroc) [dgroc@icarus ~]$ python dgroc/dgroc.py --debug
Processing project: rocketchat-desktop
Generating new source rpm for project: rocketchat-desktop
Last commit: ac8d04ec
Command to generate archive: git archive --format=tar --prefix=rocketchat-desktop/ -o/home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar HEAD
Update spec file: /home/dgroc/fedora-specs/copr/rocketchat-dev/rocketchat.spec
rocketchat-desktop: Spec already up to date
0 srpms generated
(dgroc) [dgroc@icarus ~]$ ls -l /home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar
-rw-rw-r--. 1 dgroc dgroc 7976960 May  6 14:54 /home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar
(dgroc) [dgroc@icarus ~]$ python dgroc/dgroc.py --debug
Processing project: rocketchat-desktop
Generating new source rpm for project: rocketchat-desktop
Last commit: ac8d04ec
Command to generate archive: git archive --format=tar --prefix=rocketchat-desktop/ -o/home/dgroc/rpmbuild/SOURCES/rocketchat-desktop-ac8d04ec.tar HEAD
Update spec file: /home/dgroc/fedora-specs/copr/rocketchat-dev/rocketchat.spec
rocketchat-desktop: Spec already up to date
0 srpms generated

Config is:

[main]
username = xenithorb
email = xenithorb@fedoraproject.org
copr_url = https://copr.fedoraproject.org/

[rocketchat-desktop]
git_url = https://github.com/RocketChat/Rocket.Chat.Electron.git
git_folder = /tmp/rocketchat_source/
spec_file = ~/fedora-specs/copr/rocketchat-dev/rocketchat.spec

Strangely, running it twice produces the archive - for whatever reason. I tried to demonstrate that in the above output.