Sentry.io & Heroku: error: Could not automatically determine release name - heroku

Question

Situation
Our front-end app is hosted by Heroku.
I followed this tutorial: https://docs.sentry.io/guides/integrate-frontend/
Package.json scripts:
"scripts": {
"build": "REACT_APP_RELEASE=`./node_modules/.bin/sentry-cli releases propose-version --log-level=debug` REACT_APP_STAGE=production react-app-rewired build && make setup_release",
"start": "REACT_APP_RELEASE=`./node_modules/.bin/sentry-cli releases propose-version --log-level=debug` REACT_APP_STAGE=production serve -s ./build",
... irrelevant scripts
},
Makefile
SENTRY_AUTH_TOKEN=<sentry_auth_token>
SENTRY_ORG=<sentry_org>
SENTRY_PROJECT=<sentry_project>
VERSION=`./node_modules/.bin/sentry-cli releases propose-version --log-level=debug`
PREFIX=static/js
setup_release: create_release upload_sourcemaps
create_release:
./node_modules/.bin/sentry-cli releases -o $(SENTRY_ORG) new -p $(SENTRY_PROJECT) $(VERSION) --log-level=debug
upload_sourcemaps:
./node_modules/.bin/sentry-cli releases -o $(SENTRY_ORG) -p $(SENTRY_PROJECT) files $(VERSION) \
upload-sourcemaps --url-prefix "~/$(PREFIX)" --validate build/$(PREFIX) --log-level=debug
Local test
Running npm run build locally works competely fine. Gives something like this:
Source Map Upload Report
Minified Scripts
~/static/js/0.5d07aaef.chunk.js (sourcemap at 0.5d07aaef.chunk.js.map)
~/static/js/1.baafde2d.chunk.js (sourcemap at 1.baafde2d.chunk.js.map)
...
... more entries
...
~/static/js/main.8b1cab7e.chunk.js (sourcemap at main.8b1cab7e.chunk.js.map)
~/static/js/runtime~main.b6cf63c4.js (sourcemap at runtime~main.b6cf63c4.js.map)
Source Maps
~/static/js/0.5d07aaef.chunk.js.map
~/static/js/15.5b79a308.chunk.js.map
...
... more entries
...
~/static/js/9.44f0809a.chunk.js.map
~/static/js/main.8b1cab7e.chunk.js.map
Heroku deploy test
Running the deploy sequence on Heroku gives the following:
remote: -----> Build
remote: Running build
remote:
remote: > <heroku_project_name>#0.1.0 build /tmp/build_43aa3bf4ce7530858abaf7fc0103a317
remote: > REACT_APP_RELEASE=`./node_modules/.bin/sentry-cli releases propose-version --log-level=debug` REACT_APP_STAGE=production react-app-rewired build && make setup_release
remote:
remote: INFO 2019-10-15 10:29:17.592182558 +00:00 Loaded config from /tmp/build_43aa3bf4ce7530858abaf7fc0103a317/.sentryclirc
remote: DEBUG 2019-10-15 10:29:17.592238531 +00:00 sentry-cli version: 1.48.0, platform: "linux", architecture: "x86_64"
remote: INFO 2019-10-15 10:29:17.592260636 +00:00 sentry-cli was invoked with the following command line: "/tmp/build_43aa3bf4ce7530858abaf7fc0103a317/node_modules/#sentry/cli/sentry-cli" "releases" "propose-version" "--log-level=debug"
remote: DEBUG 2019-10-15 10:29:17.592446409 +00:00 error: running update nagger
remote: DEBUG 2019-10-15 10:29:17.592465389 +00:00 skipping update nagger because session is not attended
remote: error: Could not automatically determine release name
remote: DEBUG 2019-10-15 10:29:17.592964391 +00:00 client close; no transport to shut down (from sentry)
What is this update nagger thing? Why is it causing this error?