Install / get used versions fails for checked out commit

When a module is checked out on a specific commit (detached HEAD) the scripts geusedversions/makeinstallscript fail:

An error occurred during subprocess run:
-- command: git log -n 1 --format=%H @{upstream}
-- folder: dune-istl
-- error: Command 'git log -n 1 --format=%H @{upstream}' returned non-zero exit status 128.

what is the @{upstream} doing?