Amazon web services – Lambda error uploading PIL image into S3 bucket

I am creating a Lambda function to receive a base64-encoded image in a POST request, and save it into a S3 bucket. I need to open the image in Pillow for some processing, which is irrelevant so I skipped some code. I received this error when invoking the Lambda function HTTPClientError: An HTTP Client raised an unhandled exception: sequence item 0: expected str instance, bytes found. My Lambda code is as follows:

import logging
import boto3
import base64
from io import BytesIO
from botocore.exceptions import ClientError
from PIL import Image

def lambda_handler(event, context):
    s3_bucket = 'xxx' # My S3 bucket_name
    base64_image = event.get('body', '') 
    if not base64_image:
        return {
            'statusCode': 400,
            'body': 'No image data found in the request body.'
        }
        
    image = Image.open(BytesIO(base64.b64decode(base64_image)))
    # Skipped some imaging processing code ...    

    image_bytes = BytesIO()
    image.save(image_bytes, format='PNG')
    image_bytes.seek(0)
    
    s3_client.upload_fileobj(image_bytes, s3_bucket, 'haha', ExtraArgs={'ContentType': 'image/png'})

Any help is appreciated!