Setting Mixed Schema Type Error

I've got an app that uses an older version of mongoose.js, and I've been in the process of upgrading it to 2.9.x (with the intention of upgrading to 3.x after this update is successful). It seems to (mostly) work alright, up until I try to set anything which has a schema type of Mixed. When I do this it errors out saying TypeError: Cannot read property 'name' of undefined.

Here's my schema:

mongoose = require 'mongoose'
Schema   = mongoose.Schema
Mixed   = Schema.Types.Mixed

LogItemSchema = new Schema({
    action: String
  , user: String
  , comment: String
  , time: Date
  , modal: Mixed
})

module.exports = LogItem = mongoose.model('LogItem', LogItemSchema)

I can create a new LogItem fine, up until I attempt to set the modal property (generally with a semi-complex object, ie. {type: 'string', companies: ['company1', 'company2']}), it will error out.

If I don't set modal, there are no errors. Any idea why this would be?