[2499] Vuex store defaulting to Modules mode?

I have upgraded from rc11 to @nuxtjs/next (6f6a184) and my store no longer works - Nuxt complains:

nuxt state should be a function in store/authentication/index.js

If I change state to a function in the store/authentication/index.js module then Nuxt defaults back to modules mode and as such things like a login action in my authentication module need to be called via authentication/index/login rather than just authentication/login like it should be.

I believe my store/index.js file is configured correctly for classic mode:

'use strict'

import { Store } from 'vuex'

import authentication from './authentication'

export const store = () => new Store({
  modules: {
    authentication
  },
  actions: {
    nuxtServerInit () {
      // nothing here yet
    }
  }
})