UnicodeDecodeError please help

[Frennish]

Hey!
I'm some what new to programmering and I happend to find your project really interesting but I can get it to work :/.
It work fine we I enter commands like cd and echo but it crashs when I enter commands like tree and dir with the error "output_str = str(output_bytes, "utf-8")
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 335: invalid start byte"
Whatever I can't get it working. It would awesome if you could help.
Thanks
//Frennish