UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 65534-65535: unexpected end of data

I want to encrypt file with simple AES encryption,here is my python3 source code.

import os, random, struct
from Crypto.Cipher import AES

def encrypt_file(key, in_filename, out_filename=None, chunksize=64*1024):
    if not out_filename:
        out_filename = in_filename + '.enc'
    iv = os.urandom(16)
    encryptor = AES.new(key, AES.MODE_CBC, iv)
    filesize = os.path.getsize(in_filename)
    with open(in_filename, 'rb') as infile:
        with open(out_filename, 'wb') as outfile:
            outfile.write(struct.pack('<Q', filesize))
            outfile.write(iv)
            while True:
                chunk = infile.read(chunksize)
                if len(chunk) == 0:
                    break
                elif len(chunk) % 16 != 0:
                    chunk += ' ' * (16 - len(chunk) % 16)
                outfile.write(encryptor.encrypt(chunk.decode('UTF-8','strict')))

It works fine for some files,encounter error info for some files such as below:

encrypt_file("qwertyqwertyqwer",'/tmp/test1' , out_filename=None, chunksize=64*1024)

No error info,works fine.

encrypt_file("qwertyqwertyqwer",'/tmp/test2' , out_filename=None, chunksize=64*1024)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 17, in encrypt_file
UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 65534-65535: unexpected end of data

How to fix my encrypt_file function?

Do as t.m.adam say ,to fix

outfile.write(encryptor.encrypt(chunk.decode('UTF-8','strict')))

as

outfile.write(encryptor.encrypt(chunk))

To try with some file.

encrypt_file("qwertyqwertyqwer",'/tmp/test' , out_filename=None, chunksize=64*1024)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 16, in encrypt_file
TypeError: can't concat bytes to str

from UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 65534-65535: unexpected end of data