Elixir inserting extra characters into rev-manifest.json when gulp is run

Sometimes when I run elixar versioning it inserts extra characters into the rev-manifest.json file. This causes gulp to fail when run. I've had this across a few different projects (and different versions of laravel), which makes me think maybe I'm doing something incorrectly in the gulifile.js.

Here is the rev-manifest.json file after the error has happened, everything after the first closing curly brace shouldn't be there:

{
  "js/all.js": "js/all-abfacb61c8.js"
}s"
}

And here is my gulpfile.js:

elixir(mix => {

  mix.sass(['app.scss'], 'resources/assets/css/custom.css');

  mix.styles([
      'jquery-ui.min.css',
      'bootstrap.min.css',
      'select2.min.css',
      'font-awesome.css',
      'jquery.dataTables.min.css',
      'dataTables.bootstrap4.min.css',
      'custom.css'
  ])
  .version('/css/all.css');

  mix.scripts([
        'jquery-3.1.1.min.js',
        'jquery-ui.min.js',
        'tether.min.js',
        'bootstrap.min.js',
        'select2.min.js',
        'jquery.dataTables.min.js',
        'dataTables.bootstrap4.js',
        'custom.js'
  ])
  .version('/js/all.js');

});