Crash from 'Head does not exist'

Running bors try and bors merge was working fine until, suddenly, it stopped working for the branch.
Now, I get the following error and bors crashes after creating the trying.tmp branch.

  {:error, :merge_branch, 404,
   "{\"message\":\"Head does not exist\",\"documentation_url\":\"\"}",
   {BorsNG.GitHub, :merge_branch!, 2,
    [file: 'lib/github/github.ex', line: 185]},
   {Enum, :"-reduce/3-lists^foldl/2-0-", 3,
    [file: 'lib/enum.ex', line: 2468]},
   {BorsNG.Worker.Batcher, :start_waiting_batch, 1,
    [file: 'lib/worker/batcher.ex', line: 443]},
   {BorsNG.Worker.Batcher, :poll_, 1,
    [file: 'lib/worker/batcher.ex', line: 297]},
   {BorsNG.Worker.Batcher, :handle_info, 2,
    [file: 'lib/worker/batcher.ex', line: 237]},
   {:gen_server, :try_dispatch, 4,
    [file: 'gen_server.erl', line: 1123]},
   {:gen_server, :handle_msg, 6,
    [file: 'gen_server.erl', line: 1200]},
   {:proc_lib, :init_p_do_apply, 3,
    [file: 'proc_lib.erl', line: 240]}

Any idea what is happening here?