I bet somewhere in the source there is a line which looks like: if (inclusiveEnd + 1 > size) { return ERR_INVALID; } HTTP ranges are inclusive

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.