shelljs exec (sync) problems

error.sh

#!/usr/bin/env bash
cd /tmp
git init tmp
cd tmp
git rev-parse --abbrev-ref HEAD
exit 1

index.js

console.log(require("shelljs").exec("./error.sh"));

output-mac.txt

Initialized empty Git repository in /private/tmp/tmp/.git/
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
HEAD
{ code: 1,
  output: 'Initialized empty Git repository in /private/tmp/tmp/.git/\nfatal: ambiguous argument \'HEAD\': unknown revision or path not in the working tree.\nUse \'--\' to separate paths from revisions, like this:\n\'git <command> [<revision>...] -- [<file>...]\'\nHEAD\n' }

output-travis.txt

Initialized empty Git repository in /tmp/tmp/.git/
events.js:141
      throw er; // Unhandled 'error' event
            ^
Error: write after end
    at writeAfterEnd (_stream_writable.js:158:12)
    at WriteStream.Writable.write (_stream_writable.js:207:5)
    at Socket.ondata (_stream_readable.js:524:20)
    at emitOne (events.js:82:20)
    at Socket.emit (events.js:166:7)
    at readableAddChunk (_stream_readable.js:146:16)
    at Socket.Readable.push (_stream_readable.js:109:10)
    at Pipe.onread (net.js:517:20)
shell.js: internal error
Error: Command failed: "/home/travis/.nvm/versions/io.js/v1.6.3/bin/iojs" /tmp/shelljs_39e1753021daa2512cff
    at checkExecSyncError (child_process.js:1321:13)
    at Object.execSync (child_process.js:1361:13)
    at execSync (/home/travis/build/dominykas/grunt-checkbranch/node_modules/shelljs/src/exec.js:77:11)
    at Object._exec (/home/travis/build/dominykas/grunt-checkbranch/node_modules/shelljs/src/exec.js:210:12)
    at Object.exec (/home/travis/build/dominykas/grunt-checkbranch/node_modules/shelljs/src/common.js:182:23)
    at Object.<anonymous> (/home/travis/build/dominykas/grunt-checkbranch/tmp-shelljs/run.js:1:94)
    at Module._compile (module.js:410:26)
    at Object.Module._extensions..js (module.js:428:10)
    at Module.load (module.js:335:32)
    at Function.Module._load (module.js:290:12)