nuxt: Error while initializing app TypeError: parentVal.concat is not a function

Version

2.0.0

Reproduction link

https://nuxtjs.org

Steps to reproduce

Upgrade a project to Nuxt 2.0.0.

What is expected ?

No error.

What is actually happening?

Hello. I get the following error that is not easy to understand and debug:

[nuxt] Error while initializing app TypeError: parentVal.concat is not a function
    at mergeHook (commons.app.js:10224)
    at mergeField (commons.app.js:10485)
    at mergeOptions (commons.app.js:10476)
    at Function.Vue.extend (commons.app.js:13793)
    at sanitizeComponent (app.js:4688)
    at _callee$ (app.js:4757)
    at tryCatch (commons.app.js:4154)
    at Generator.invoke [as _invoke] (commons.app.js:4388)
    at Generator.prototype.(anonymous function) [as next] (http://localhost:3000/_nuxt/commons.app.js:4206:21)
    at asyncGeneratorStep (commons.app.js:33)

It seems to be part of the Vue.js framework. Any idea what could cause this in my code? A module, a Vue plugin, an external lib?

<div align="right">_{This bug report is available on Nuxt community (#c7819)}</div>