I really doubt that tests fail due to locale, this should not matter on Codewars (but maybe it does for some strange reason, but I cannot check at the moment). I suspect that at some point you call double.Parse with something strange, like an empty string, or an incorrectly processed string.

Take a close look at the line 97 (commented as //Alternative final) and see what it tries to parse. From my (short) tests it seems that it can get some invalid strings to parse.

After more than 2 years...
Yes that's true. Seems like the tests didn't cover that.