I'm performing mocha tests on my React components that I wrote using ES6 syntax. I'm using Istanbul to do code coverage tests. When i set my `NODE_ENV` to 'test`, I get the following output:

```
----------|----------|----------|----------|----------|----------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
----------|----------|----------|----------|----------|----------------|
All files |  Unknown |  Unknown |  Unknown |  Unknown |                |
----------|----------|----------|----------|----------|----------------|

```

As can be seen, it says unknown for everything. When I remove the `NODE_ENV` variable, it works fine. What can I do to run this test in the above environment?