I bet somewhere in the source there is a line which looks like:

    if (inclusiveEnd + 1 > size) {
        return ERR_INVALID;
    }

HTTP ranges are inclusive, and most likely implemented here with unsigned 64 bit integers. My guess is the author converted to exclusive range, then compared with size, as a form of validation. It passes the check, because 18446744073709551615 + 1 results in wraparound to 0.

The general solution is instead to use something like:

    if (size < offset || start > size - offset) {
        ... // range violated
    }

But you hardly ever see people do that.