Coverage file is empty for Chrome but not for Firefox

I'm setting up coverage with Karma and was beating my head against the wall when I saw in the DEBUG log that files get preprocessed, the coverage file gets written out but it is empty.

Then after some time I just switched from my Chrome to Firefox and if I run the tests with Firefox then the coverage file is populated.

Let me know if you need any more information or maybe this will just serve to somebody else as an idea what to try when nothing else works.