isocalendar giving me some wrong week number

Question

Here's my dataframe :

And here's my code :

week_number= []

for date in df['date']:
    a_date = date
    print(a_date)
    dt = datetime.datetime.strptime(a_date, '%Y-%m-%d')
    print(dt)
    number = dt.isocalendar()[1]
    print(number)
    week_number.append(number)

And here's the output :

2019-11-14
2019-11-14 00:00:00
46

2019-12-17
2019-12-17 00:00:00
51

2019-12-07
2019-12-07 00:00:00
49

2019-12-14
2019-12-14 00:00:00
50

2019-12-30
2019-12-30 00:00:00
1

As you can see, the last one give me 1, why did I do wrong ?