[1009] Nuxt generate won't finish when there is mode: 'hash' in router

To reproduce

vue init nuxt/starter router.mode
cd router.mode;npm i

add router: { mode: 'hash' }, to nuxt.config.js then npm run generate

⫸  npm run generate

> router.mode@1.0.0 generate /Users/wao/Projects/router.mode
> nuxt generate

[nuxt] Generating...
  nuxt:build App root: /Users/wao/Projects/router.mode +0ms
  nuxt:build Generating /Users/wao/Projects/router.mode/.nuxt files... +1ms
  nuxt:build Generating files... +24ms
  nuxt:build Generating routes... +31ms
  nuxt:build Building files... +72ms
Build completed in 7.53s

[nuxt:build:client]
 Hash: d40647ac3d151e6fc802
Version: webpack 2.6.1
Time: 7534ms
                                Asset       Size  Chunks             Chunk Names
0.nuxt.bundle.39a9a9f2f9eff960c6c0.js    7.65 kB       0  [emitted]  pages/index
1.nuxt.bundle.01cb7331534f6ba0471e.js    2.79 kB       1  [emitted]  layouts/default
vendor.bundle.6743fffbb1eeceb23dde.js     135 kB       2  [emitted]  vendor
  nuxt.bundle.5357e85de24767dadc0e.js    23.9 kB       3  [emitted]  app
     manifest.d40647ac3d151e6fc802.js    1.51 kB       4  [emitted]  manifest
                           index.html  132 bytes          [emitted]
                 client-manifest.json    5.54 kB          [emitted]
[nuxt:build:server]
 Hash: a24e14f4c74d9a412fe6
Version: webpack 2.6.1
Time: 774ms
             Asset    Size  Chunks             Chunk Names
server-bundle.json  100 kB          [emitted]
  nuxt:generate Destination folder cleaned +9s
  nuxt:generate Static & build files copied +16ms
TypeError: Cannot read property 'forEach' of undefined
    at decorateWithPayloads (/Users/wao/Projects/router.mode/node_modules/nuxt/dist/nuxt.js:1330:29)
    at Nuxt._callee2$ (/Users/wao/Projects/router.mode/node_modules/nuxt/dist/nuxt.js:1430:20)
    at tryCatch (/Users/wao/Projects/router.mode/node_modules/regenerator-runtime/runtime.js:65:40)
    at GeneratorFunctionPrototype.invoke [as _invoke] (/Users/wao/Projects/router.mode/node_modules/regenerator-runtime/runtime.js:303:22)
    at GeneratorFunctionPrototype.prototype.(anonymous function) [as next] (/Users/wao/Projects/router.mode/node_modules/regenerator-runtime/runtime.js:117:21)
    at step (/Users/wao/Projects/router.mode/node_modules/babel-runtime/helpers/asyncToGenerator.js:17:30)
    at /Users/wao/Projects/router.mode/node_modules/babel-runtime/helpers/asyncToGenerator.js:28:13
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! router.mode@1.0.0 generate: `nuxt generate`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the router.mode@1.0.0 generate script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/wao/.npm/_logs/2017-07-01T22_07_00_879Z-debug.log

⫸  node -v
v6.11.0
⫸  npm --version
5.0.4
⫸  grep 'version' node_modules/nuxt/package.json
  "version": "1.0.0-alpha.4"

Tips?