can't cover with mocha when using mock-fs

[fiws]

Hi I am using this command istanbul cover _mocha -- -u exports -R spec test/**/*.js

and getting this error:

 "stack": [
   "Error: ENOENT, no such file or directory '/home/fiws/npm/lib/node_modules/istanbul/lib/assets'",
   "    at Binding.<anonymous> (/home/fiws/project/node_modules/mock-fs/lib/binding.js:604:13)",
   "    at maybeCallback (/home/fiws/project/node_modules/mock-fs/lib/binding.js:43:17)",
   "    at Binding.readdir (/home/fiws/project/node_modules/mock-fs/lib/binding.js:596:10)",
   "    at Object.fs.readdirSync (/home/fiws/project/node_modules/mock-fs/node/fs-6.3.0.js:909:18)",
   "    at Object.readdirSync (/home/fiws/project/node_modules/mock-fs/lib/index.js:59:31)",
   "    at copyAssets (/home/fiws/npm/lib/node_modules/istanbul/lib/report/html.js:543:20)",
   "    at /home/fiws/npm/lib/node_modules/istanbul/lib/report/html.js:562:13",
   "    at Array.forEach (native)",
   "    at HtmlReport.writeReport (/home/fiws/npm/lib/node_modules/istanbul/lib/report/html.js:561:26)",
   "    at LcovReport.writeReport (/home/fiws/npm/lib/node_modules/istanbul/lib/report/lcov.js:55:19)",
   "    at /home/fiws/npm/lib/node_modules/istanbul/lib/reporter.js:93:20",
   "    at Array.forEach (native)",
   "    at Object.write (/home/fiws/npm/lib/node_modules/istanbul/lib/reporter.js:87:30)",
   "    at process.<anonymous> (/home/fiws/npm/lib/node_modules/istanbul/lib/command/common/run-with-cover.js:249:30)",
   "    at process.g (events.js:286:16)",
   "    at emitOne (events.js:96:13)",
   "    at process.emit (events.js:188:7)",
   "    at process.exit (internal/process.js:146:15)",
   "    at done (/home/fiws/npm/lib/node_modules/mocha/bin/_mocha:416:32)",
   "    at afterWrite (_stream_writable.js:361:3)",
   "    at _combinedTickCallback (internal/process/next_tick.js:80:20)",
   "    at process._tickCallback (internal/process/next_tick.js:98:9)"
 ],
 "level": "error",
 "message": "uncaughtException: ENOENT, no such file or directory '/home/fiws/npm/lib/node_modules/istanbul/lib/assets'",
 "timestamp": "2016-08-30T07:51:25.280Z"

and the process hangs.
Seems to be related to mock-fs used by the tests..

[fiws]

probably project related.

[simeg]

Are you calling mock.restore() in the tear down method? See the first example in the mock-fs README and this issue.