CSV file upload from buffer to S3

I am trying to upload content taken out of a model in Django as a csv file. I don't want to save the file locally, but keep it in the buffer and upload to s3. Currently, this code does not error as is, and uploads the file properly, however, the file is empty.

file_name='some_file.csv'
fields = [list_of_fields]
header = [header_fields]

buff =  io.StringIO()
writer = csv.writer(buff, dialect='excel', delimiter=',')
writer.writerow(header)
for value in some_queryset:
    row = []
    for field in fields:
        # filling in the row
    writer.writerow(row)

# Upload to s3
client = boto3.client('s3')
bucket = 'some_bucket_name'
date_time = datetime.datetime.now()
date = date_time.date()
time = date_time.time()
dt = '{year}_{month}_{day}__{hour}_{minute}_{second}'.format(
    day=date.day,
    hour=time.hour,
    minute=time.minute,
    month=date.month,
    second=time.second,
    year=date.year,
)
key = 'some_name_{0}.csv'.format(dt)

client.upload_fileobj(buff, bucket, key)

If I take the buffer's content, it is definitely writing it:

content = buff.getvalue()
content.encode('utf-8')
print("content: {0}".format(content)) # prints the csv content

EDIT: I am doing a similar thing with a zip file, created in a buffer:

with zipfile.ZipFile(buff, 'w') as archive:

Writing to the archive (adding pdf files that I am generating), and once I am done, I execute this: buff.seek(0) which seems to be necessary. If I do a similar thing above, it will error out: Unicode-objects must be encoded before hashing