Not supported environment warning when deploying via GitLab CI

Summary

When I'm deploying my application via GitLab CI I'm getting the following warning message:

WARNING: environment is not supported by selected executor and shell

The deployment itself is getting done succesfully?!

Steps to reproduce

My .gitlab-ci.yml looks like this:

image: php:5.6

cache:
  key: $CI_BUILD_REF_NAME
  paths:
  - .composercache

before_script:
  - bash ci/before_script.sh > /dev/null

test:5.6:
  image: php:5.6
  stage: test
  script:
    - php ./vendor/bin/phpunit -c ./tests/phpunit.dist.xml

test:7.0:
  image: php:7.0
  stage: test
  script:
    - php ./vendor/bin/phpunit -c ./tests/phpunit.dist.xml

deploy:production:
  stage: deploy
  environment: production
  script:
    - vendor/bin/dep --file=./ci/deploy/deploy.php deploy production --tag $CI_BUILD_TAG
  only:
    - tags

Expected behavior

No warning is displayed

Actual behavior

The aforementioned warning message is displayed.

The deployment itself is progressing flawlessly, so the warning message is just a little bit annoying.

Can anyone help me what the problem is?