"yaml invalid" in case if `variables` is set to multiline key value

Summary

It's impossible to set variables to key-value hash - always error 'yaml invalid' with message 'variables config should be a hash of key value pairs' is thrown.

Steps to reproduce

Just try to add this to .gitlab-ci.yml:

variables:
    GIT_STRATEGY: fetch
    GIT_CHECKOUT: false

While it is one-liner (I mean, only one parameter is set) - it works. At the time you add another line - it is broken.

Example Project

https://gitlab.com/PatchRanger/bugshowcase-gitlab-36401

Here is the failed build with exact the same error message could be found: https://gitlab.com/PatchRanger/bugshowcase-gitlab-36401/pipelines .

What is the current bug behavior?

The pipeline is broken by YAML-validation, which is incorrect as aforementioned structure is valid YAML syntax.

What is the expected correct behavior?

Pipeline should pass YAML-validation and run.

Output of checks

As we are using Gitlab.com - it happens at gitlab.com.

Possible fixes

Looks like it is related to false keyword: in case if I surround false by quotes ("false") - it passes validation.