GitLab CI YAML Parser doesn't allow intentionally insecure variables

When adding an unhashed, intentionally insecure variable to .gitlab-ci.yml, GitLab CI complains: variables config should be a hash of key value pairs. However, this should be supported and there is an example in the docs. I followed the same syntax:

variables:
  - DEP1_BRANCH: "master"
  - DEP2_VERSION: "1.2.3"