this.isSelected is not a function error in Mongoose.js

I am receiving the following error within Mongoose.js. It doesn't happen all the time, just sometimes.

TypeError: this.isSelected is not a function 
    at model.Model.$__version (/app/node_modules/mongoose/lib/model.js:589:13) 
    at model.Model.$__delta (/app/node_modules/mongoose/lib/model.js:511:10) 
    at model.Model.$__handleSave (/app/node_modules/mongoose/lib/model.js:147:22) 
    at model.Model.$__save (/app/node_modules/mongoose/lib/model.js:185:9) 
    at model.Model.save (/app/node_modules/mongoose/lib/model.js:283:15) 
    at model._done (/app/node_modules/mongoose/node_modules/hooks-fixed/hooks.js:101:24) 
    at _next (/app/node_modules/mongoose/node_modules/hooks-fixed/hooks.js:64:28) 
    at fnWrapper (/app/node_modules/mongoose/node_modules/hooks-fixed/hooks.js:186:8) 
    at model.Object.defineProperty.value.fn (/app/node_modules/mongoose/lib/schema.js:221:11) 
    at _next (/app/node_modules/mongoose/node_modules/hooks-fixed/hooks.js:62:30) 
    at fnWrapper (/app/node_modules/mongoose/node_modules/hooks-fixed/hooks.js:186:8) 
    at /app/node_modules/mongoose/lib/schema.js:196:17 
    at /app/node_modules/mongoose/node_modules/kareem/index.js:127:16 
    at wrapped (/app/node_modules/newrelic/lib/transaction/tracer/index.js:161:28) 
    at nextTickCallbackWith0Args (node.js:420:9) 
    at process._tickDomainCallback [as _tickCallback] (node.js:390:13)

The relevant code that I believe is producing that error is as follows:

exports.updateTheme = function(req, res) {
  var theme = req.theme;
  delete req.body.created;
  delete req.body.modified;
  delete req.body.user;
  delete req.body._id;

  theme.modified = new Date().getTime();
  theme = _.extend(theme, req.body);
  theme.markModified('copyMinNumericalAttsColorNodes');
  theme.markModified('copyMaxNumericalAttsColorNodes');
  theme.save(function(err, theme) {
    if(err) {
      console.log(err);
      res.status(500).send(err);
    } else {
      res.json(theme);
    }
  });
};

I would really appreciate any thoughts on what could be producing this error. Thanks!

via Andrew