"Can't set headers after they are sent" error being thrown but no double callbacks

My code is in two files: router.js and work.js

Here is the code for work.js:

getThumbnailItems = function(callback, res) {
        fs.readdir(this.thumbnailPath, function(err, files) {

                callback(files, res);
            }
        });
    }

Here is where the method is called in router.js:

exports.home = function(req, res) {
    getThumbnailItems(function(items) {
        console.log(items);
        if (res) {
            res.render("home", {title: "The Work Speaks For Itself", items: items});
        }
    }, res)

}

I am using a callback in getThumbnails to render the home view. But an error "Can't set headers after they are sent" keeps getting thrown. Does anyone know what is going on here?