Sorry, the error went away by itself apparently!
...huh, I mean, in a certain sense I'm not sure how it could have happened in the first place (I'll get to that below), but I'm glad it's working for now.
As for what was happening, the code that was hitting the bug was the part that reads back characters that have already been printed to the terminal (the reason why the code does this is so that it can join the two corners of windows together, like it does on the bottom left and bottom right, in order to make it look like they're connected). However, somehow when it read back the character, it found neither a line-drawing character nor a valid unicode symbol, which is bizarre since those are the only two things that should have been sent to the terminal already. Actually, the really bizarre thing is that the result from curses is already bitwise anded with 0x4000ff, which means that the argument to chr() couldn't possibly have been what the above error indicates that it was.
In any case, if I ever reproduce (and fix) this I'll be sure to add an explanation of what happened in this thread.