Python 3 error: float division by zero in reporthook

I'm trying to make a simple code that downloads 4365 files (.mp4, .wmv, .jpeg & .pdf) from a .json file, the download part its done, but I wanted to get a reporthook telling me the %, Mb, speed an duration.

Even though sometimes it runs, I'm getting errors:

file "dw.py", line 21, in reporthook
    speed = int(progress_size / (1024 *  duration))
ZeroDivisionError: float division by zero

This is my code:

import urllib.request
import json
import sys
import time

with open('finalsinbin.json') as json_data:   # importar toda la lista de videos 
items = json.load(json_data)

def reporthook(count, block_size, total_size):
    global start_time
    if count == 0:
        start_time = time.time()
        return
    duration = time.time() - start_time
    progress_size = int(count * block_size)
    speed = int(progress_size / (1024 * duration))
    percent = min(int(count*blockSize*100/totalSize),100)
    sys.stdout.write("\r...%d%%, %d MB, %d KB/s, %d seconds passed" % (percent, progress_size / (1024 * 1024), speed, duration))
    sys.stdout.flush()


def batch(startAt, stopAt): 
   a=0 
   for i in items:
        a+=1

    if a < startAt or a > stopAt:
        continue

    num = i['id']
    url = i['url']
    typ = i['type']
    urlfinal = (url + "/" + str(num) + typ)
    filename = (str(num) + typ)
    print( '[%d] Descargando el archivo %s...' % (a, str(num) + typ))
    urllib.request.urlretrieve(urlfinal, filename, reporthook)  

batch(1, 100) #download file from n to N

Try to use time.perf_counter() instead of time.time(). At least in my environment, time.perf_counter() never went to 0 as below.

import time

def time_perf_counter():
    start_time = time.perf_counter()
    return time.perf_counter() - start_time

def time_time():
    start_time = time.time()
    return time.time() - start_time

def test_timer(timer, n):
    count = 0
    for i in range(n):
        if timer() == 0:
            count += 1
    return count

n_test = 100000

print('{:>6s} / {:>6s}'.format(
    '# fail',
    '# test'))
print('{:6d} / {:6d}'.format(
    test_timer(time_perf_counter, n_test),
    n_test))
print('{:6d} / {:6d}'.format(
    test_timer(time_time, n_test),
    n_test))

The results are, for example:

# fail / # test
     0 / 100000
 89109 / 100000

My environments: MBP High Sierra, Python 3.6.3