express serve partials from outside tree

index.js

#!/usr/bin/env node

const app = require('express')();
const viewsDir = __dirname + '/views';
const otherDir = __dirname + '/../ax';
const hbs = require('express-handlebars').create({
  defaultLayout: 'main',
  layoutsDir: viewsDir + '/layouts',
  partialsDir: [
    viewsDir + '/partials',
    otherDir
  ],
  extname: '.hbs'
});

app.engine('.hbs', hbs.engine);
app.set('view engine', '.hbs');
app.set('views', viewsDir);

app.get('/', function (req, res) {
  res.render('index');
});

app.listen(3000, function () {
  console.log('listening on: 3000');
});

../ax/header.hbs is outside the tree, views/index.hbs has:

{{> header}}

in it and it renders properly.