Why doesn't ember-cli see my vendor file

I have an external javascript file that I've placed in vendor/. In my ember-cli-build.js file, I import the script:

var EmberApp = require('ember-cli/lib/broccoli/ember-app')

module.exports = function (defaults) {
  var app = new EmberApp(defaults, {
    // options
  })

  app.import('vendor/myscript.js')

  return app.toTree()
}

When I run the app I get this warning: Warning: ignoring input sourcemap for vendor/myscript.js because ENOENT: no such file or directory, open '/path/to/project/root/tmp/source_map_concat-input_base_path-http://ift.tt/2hF4pbZ'

Why is it searching for the script in /tmp?