istanbul + mocha + chai 跑覆盖率的时候提示`describe is not defined`

Question

RT，我在全局跟当前项目都安装了5.0.0版本的mocha以及最新版本istanbul，但是不管是跑
./node_modules/.bin/istanbul cover ./test/index.test.js还是istanbul cover ./test/index.test.js都提示我

/Users/hubery/workspace/repos/torch/test/index.test.js:4
describe('the basic usage of Promise', function() {
^

ReferenceError: describe is not defined
    at Object.<anonymous> (/Users/hubery/workspace/repos/torch/test/index.test.js:4:1)
    at Module._compile (module.js:635:30)
    at Object.Module._extensions..js (module.js:646:10)
    at Object.Module._extensions.(anonymous function) [as .js] (/Users/hubery/workspace/repos/torch/node_modules/istanbul/lib/hook.js:109:37)
    at Module.load (module.js:554:32)
    at tryModuleLoad (module.js:497:12)
    at Function.Module._load (module.js:489:3)
    at Function.Module.runMain (module.js:676:10)
    at runFn (/Users/hubery/workspace/repos/torch/node_modules/istanbul/lib/command/common/run-with-cover.js:122:16)
    at /Users/hubery/workspace/repos/torch/node_modules/istanbul/lib/command/common/run-with-cover.js:251:17

请问如何解决？