Error 500 accessing branches via API

https://sentry.gitlap.com/gitlab/gitlabcom/issues/110249/

TypeError: no implicit conversion of nil into String
  lib/gitlab/git/popen.rb:21:in `popen'
    Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
  lib/gitlab/git/repository.rb:1111:in `block in run_git'
    popen([Gitlab.config.git.bin_path, *args], path, env)
  lib/gitlab/git/storage/circuit_breaker.rb:57:in `perform'
    return yield unless enabled?
  lib/gitlab/git/repository.rb:1110:in `run_git'
    circuit_breaker.perform do
  lib/gitlab/git/repository.rb:1225:in `git_merged_branch_names'
    lines = run_git(['branch', '--merged', root_ref] + branch_names)
...
(125 additional frame(s) were not displayed)

TypeError: no implicit conversion of nil into String