yo webapp 项目运行grunt命令报错，貌似是mocha

Question

Ming@Ming-PC MINGW64 ~/Source
$ grunt
Running "newer:eslint" (newer) task

Running "newer:eslint:target" (newer) task

Running "eslint:target" (eslint) task

Running "newer-postrun:eslint:target:1:C:\Users\Ming\Source\node_modules\grunt-n ewer\.cache" (newer-postrun) task

Running "test" task

Running "clean:server" (clean) task

1 path cleaned.

Running "concurrent:test" (concurrent) task

Running "babel:dist" (babel) task

Running "babel:test" (babel) task

Done, without errors.

Execution Time (2016-01-14 14:07:32 UTC)
loading tasks 349ms ███████████ 24%
loading grunt-babel 836ms █████████████████████████ 59%
babel:dist 190ms ██████ 13%
babel:test 51ms ██ 4%
Total 1.4s

Running "postcss:dist" (postcss) task

Running "browserSync:test" (browserSync) task

Running "mocha:all" (mocha) task
Testing: http://localhost:9001/index.html
Warning: Cannot read property 'pid' of undefined Use --force to continue.

Aborted due to warnings.