Python Error: can't concat str to bytes when using struct.csc32

I have this Python code to process and output a PNG image for CTF (Catch The Flag). The original code was in Python 2.7, and I did some changes to make it work in Python 3

However when running it, I have this error:

f.write(struct.pack('>L', crc32(chtype+paldata)&0xffffffff))

TypeError: can't concat str to bytes

The code:

#!/usr/bin/env python

import sys
import struct
from zlib import crc32
import os

# PNG file format signature
pngsig = b'\x89PNG\r\n\x1a\n'

def swap_palette(filename, n):
    # open in read+write mode
    with open(filename, 'r+b') as f:
        f.seek(0)
        # verify that we have a PNG file
        if f.read(len(pngsig)) != pngsig:
            raise RuntimeError('not a png file!')

        while True:
            chunkstr = f.read(8)

            if len(chunkstr) != 8:
                # end of file
                break

            # decode the chunk header
            length, chtype = struct.unpack('>L4s', chunkstr)
            # we only care about palette chunks
            print(chtype)
            if chtype == b'PLTE':
                print("I'm here")
                curpos = f.tell()
                paldata = f.read(length)
        # replace palette entry n with white, the rest with black
                paldata = ("\x00\x00\x00" * n) + "\xff\xff\xff" + ("\x00\x00\x00" * (256 - n - 1))
        # replace palette entry 127 to 127 + n with white, the rest with black
                #paldata = ("\x00\x00\x00" * 127) + ("\xff\xff\xff"*n) + ("\x00\x00\x00" * (256 - (127 + n)))

                # go back and write the modified palette in-place
                f.seek(curpos)
                f.write(os.fsencode(paldata))
                f.write(struct.pack('>L', crc32(chtype+paldata)&0xffffffff))
            else:
                # skip over non-palette chunks
                f.seek(length+4, os.SEEK_CUR)

if __name__ == '__main__':
    import shutil
    shutil.copyfile(sys.argv[1], sys.argv[2])
    swap_palette(sys.argv[2], int(sys.argv[3]))

The cmd command should be: python [filename.py] "[png image path]" "[output path].png" [any integer between 0 to 127]

The result should a new black image with some white areas within it