dotenv cause karma test failed

When I use require('dotenv').config() in karmar.conf.js to test Angularjs, got this error:

Uncaught Error: [$injector:nomod] Module 'app' is not available! You either misspelled the module name or forgot to load it. If registering a module ensure that you specify the dependencies as the second argument.

But if delete this line, everything seems OK.

It's hard to figure it out. Does anybody encounter this situation?