CI_SERVER_REVISION=fatal: Not a git repository (or any of the parent directories): .git

When using Gitlab 10.7.3 for CI I have noticed that when I capture all of the CI_ variables sent to the runner that CI_SERVER_REVISION is populated with an error trying to access a host or repository that doesn't exist.

Before I classify this as a bug, would someone be able to expand upon where this variable actually gets its information from?
At the moment I take the CI variables and source them on a different hostname for specific purposes, but this throws an error upon sourcing, and instead of just building a workaround, I wanted to try to understand how it is being caused in the first place.