Mongoose error on creating Model (using Step)

When trying to create a model in Mongoose I get the following error

[TypeError: Cannot read property 'options' of undefined]

I have no idea what's causing it

"use strict";
var Step = require('step');
var mongoose = require('mongoose');
var Schema = mongoose.Schema;

function randomFunction() {
    var categorySchema = new Schema({
        id: Number,
        name: String,
        description: String
    }, { collection: 'categories' });

    var Category;

    //...

    mongoose.connect('mongodb://localhost/grouping');

    new Step(
        function() { //Connect to mongodb
            var db = mongoose.connection;
            db.on('error', console.error.bind(console, 'connection error:'));
            db.on('open', this);
        },
        function() {  //Create model
            console.log(categorySchema); //Logs the schema object right
            Category = mongoose.Model('Category', categorySchema);


        },
        function(err) {
            console.log(err);  //Error here
        });
    //...
}

I'm very new to Mongo (and fairly new to node) but I have absolutely no idea what the error message means.

I know I have options defined in the schema but I cant see how it would be undefined, can anyone point me in the right direction?

Note - this is a big cut out of the original code, this is the general structure (there's actually some code below mongoose.Model('Cat... but it gets skipped, I assume because the error is thrown by the mongoose.Model call as not even a console.log("Hello"); is printed straight after it.

EDIT I've found that inside Mongoose (mongoose/lib/document.js) tries to get this.schema but it's undefined

function Document (obj, fields, skipId) { //Line 37
    this.$__ = new InternalCache;
    this.isNew = true;
    this.errors = undefined;

    var schema = this.schema; //-> undefined
    // ...