Well, that was embarrassing. It's fixed now.

(If you're interested, this was the bug: the test code called reserve() on the vector, but didn't set its actual size. So solutions that ignore pixels.size() and just try to access the data anyway -- which was all of them, until you came along -- worked just fine.)

The test suite calls average multiple times. Your implementation should be able to handle that.

Not an issue. This is what the label Question is for.