mocha API route test fails

I receive the following error from running my mocha test spec:

1) Song API "before all" hook:
     TypeError: Undefined type `undefined` at `paths._id`
  Did you try nesting Schemas? You can only nest using refs or arrays.
      at Function.Schema.interpretAsType (node_modules/mongoose/lib/schema.js:599:11)
      at Schema.path (node_modules/mongoose/lib/schema.js:507:29)
      at Schema.add (node_modules/mongoose/lib/schema.js:391:12)
      at Schema.add (node_modules/mongoose/lib/schema.js:386:14)
      at new Schema (node_modules/mongoose/lib/schema.js:94:10)
      at Object.<anonymous> (src/server/models/songModel.js:29:18)
      at module.exports (src/server/models.js:12:26)
      at Context.<anonymous> (src/server/router.spec.js:16:31)

  2) Song API "after all" hook:
     TypeError: Cannot read property 'close' of undefined
      at Context.<anonymous> (src/server/router.spec.js:27:9)

My theory is that I cannot nest arrays within arrays as I've done in my schema. Is this correct? To me the docs say it's ok...for reference, my router.spec.js:

var assert = require('assert'),
    express = require('express'),
    superagent = require('superagent'),
    wagner = require('wagner-core');

var URL_ROOT = 'http://localhost:3000';

describe('Song API', function() {
    var server,
        Song;

    before(function() {
        var app = express();

        //bootstrap server
        models = require('./models')(wagner);
        app.use(require('./router')(wagner));

        server = app.listen(3000);

        //make Song model available for tests
        Song = models.Song;
    });

    after(function() {
        //shut the server down when we're done
        server.close();
    });

    beforeEach(function(done) {
        //Make sure songs are empty before wach test
        Song.remove({}, function(error) {
            assert.ifError(error);
            done();
        });     
    });

    it('can load all songs', function(done) {
        //Create a single song
        Song.create({ _id: 'Test_Song'}, function(error, doc) {
            assert.ifError(error);
            var url = URL_ROOT + '/songs';
            //make an http request to the url
            superagent.get(url, function(error, res) {
                assert.ifError(error);
                var result;
                //Make sure we received a song called "test song" back
                assert.doesNotThrow(function() {
                    result = JSON.parse(res.text);
                });
                assert.ok(result.song);
                assert.equal(result.song._id, 'Test_Song');
                done();
            });
        });
    });

and my schema:

var songSchema = new mongoose.Schema({
    _id: {type: String},
    createdBy: String,
    modifiedBy: String,
    instruments: [{
        id: Number,
        member: String,
        type: String
    }],
    sections: [{
        name: String,
        bpm: Number,
        timeSig: Number,
        measures: [{
            id: Number,
            rows: [{
                chord: String,
                blocks: [{
                    id: Number,
                    isChecked: Boolean
                }]
            }]
        }]
    }] 
});

module.exports = new mongoose.Schema(songSchema);
module.exports.songSchema = songSchema;