Need help identify error: unsupported operand type(s) for +: '_hashlib.HASH' and 'bytes'

Question

Below is a part of my code

import hashlib
from Crypto.Cipher import AES
from Crypto import Random

...

  def pad(s):
        # Add \x00 (byte) to the end of s to make the len is multiply of AES.block_size
        return s + b"\0"*(AES.block_size - len(s) % AES.block_size)


  def encrypt(key,plaintext):
        # padding original text
        padtext = pad(plaintext)
        
        # Choose a random, AES.block_size.
        iv = Random.new().read(AES.block_size)

        # Create AES-CBC cipher based on key and iv
        cipher = AES.new(key, AES.MODE_CBC, iv)
        
        # Encrypt pad text
        en_cipher = cipher.encrypt(padtext)
        
        # Create checksum
        cs = hashlib.md5(en_cipher).digest()

        return cs + iv + en_cipher

It runs on my laptop, but when I run it on another PC I got this error:

return cs + iv + en_cipher
TypeError: unsupported operand type(s) for +: '_hashlib.HASH' and 'bytes'

I think I miss to install a package but I am not sure what it is. I cannot find this error anywhere.