Sorry to go on about this, but actually I now see why it _might_ not be considered a bug. The UTF-16 sequence corresponding to "Anastasia" in Greek letters contains no embedded nul byte, since each of the Greek letters requires 2 non-empty bytes for its representation. But the appended ASCII characters will each be represented by a single "active" byte followed by a nul. (UTF-16 requires at least two bytes for each character, and pads with nuls as needed.)

So I think what R's error message is trying to say is that the result of conversion doesn't qualify as a string, where "string" means a sequence of bytes _terminated_ by a nul byte.