Unicode characters

[mallo-c]

There is what I have entered:

Encrypted Text :ff9c
Key :aBcD
Traceback (most recent call last):
  File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 31, in <module>
    start(fakepyfile,mainpyfile)
  File "/data/user/0/ru.iiec.pydroid3/files/accomp_files/iiec_run/iiec_run.py", line 30, in start
    exec(open(mainpyfile).read(),  __main__.__dict__)
  File "<string>", line 19, in <module>
  File "<string>", line 7, in cipherDe
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start

But I expected a replacement byte (0xFFFD)
Can you fix it?