Env CIRCLE_USERNAME is null

It seems like it’s null all the time. I need it during a build triggered by a push to a named branch. The documentation seems to suggest that it should contain the github username of the user that pushed the branch, but during my builds it always seems to be null. e.g.:

https://circleci.com/gh/mozilla/bedrock/117

Still see this issue. Perhaps I’m reading the documentation wrong on how this variable is supposed to be set and used, but I could really use this information (the github username of the user that pushed the branch or restarted the build via the UI) during certain builds. Ideas anyone?