Handlebars / Assemble.io - Handing return data from helper

I have a handlebars helper I want to parse fixture data and then iterate through that data with your standard handlebars {{#each}} helper. Mind you, I'm using Assemble.io, so that may impact things.

Heres my helper:

Handlebars.registerHelper('parseFixture', function(path, options) {
    if (!path || typeof path !== 'string') { return false; }

    var fs = require('fs');
    var nodePath = require('path');

    path = nodePath.join(__dirname, '../fixtures/' + path);

    fs.readFile(path, function(err, data) {
        if (err) {
            console.error('ERROR: ', err);
            return err;
        }

        data = data.toString('utf8')
        data = JSON.parse(data);

        return data;
    })
});

And In my template:

<nav>

    <ul>
        {{#parseFixture 'header.json'}}
            {{nav}}
            {{#each nav-bar.nav-links}}
                <li><a href="javascript:">{{this.topLevel}}</a></li>
            {{/each}}
        {{/parseFixture}}
    </ul>

</nav>

And the fixture data I'm using:

{
    "nav": [
        {
            "topLevel": "Things to Do"
        }, {
            "topLevel": "Places to Stay"
        }, {
            "topLevel": "Where to Eat"
        },{
            "topLevel": "Events"
        }, {
            "topLevel": "Deals"
        }, {
            "topLevel": "Plan Your Trip"
        }
    ]
}

So, I'm retrieving the fixture data properly, and it parses fine. If I log it out right above the return statement everything is grand. But even trying to output {{nav}} in the template does nothing.

Another weird thing thats probably either something with my grunt server or assemble.io: I only see the logged output when I completely shutdown and restart the server.

Any ideas on what I'm doing wrong?

Edit: I think the problem is that I'm using readFile() and not readFileSync(). I changed it however and I get:

Warning: Bad arguments

It doesnt give me anything more descriptive, but I can follow added console.log()s right up to the readFileSync().

Does readFileSync() screw up grunt server or assemble?