I am able to use Karma, Tape, and Istanbul (specifically, babel-istanbul) to get what look like correct coverage reports of my ES6 codebase, but the HTML report shows some covered lines in red, even as it shows in the margin that they have been covered some number of times in the test suite:

[![enter image description here](https://i.stack.imgur.com/8LEQy.png)](https://i.stack.imgur.com/8LEQy.png)

In particular, line 13 shows 4x coverage, which reflects the test I just wrote (which calls `concat()` four times).

Why is that line red?