错误：时间数据与格式不匹配？

Question

这是我的python代码：

from itertools import groupby
from operator import itemgetter
from datetime import datetime
import locale
o = open('F6.txt','w')
i = 0
count = 0
try:
    reviews = []
    tosort = []
    with open('fileaa','rb') as f:
        for line in f:
            line.strip()
            line2 = line.split('\t')
            if(len(line2) == 8):
                reviews.append(line2)
            else:
                count+=1
        print len(reviews)
        reviews.sort(key=itemgetter(1))
        for key, group in groupby(reviews, key=itemgetter(1)):
            i = 0
            tosort = []
            for value in group:
                tosort.append(value)
            tosort.sort(key = lambda x: datetime.strptime(x[2],'%B %d, %Y'))
            #tosort.sort(key = lambda x: x[2].split(' '))
            for line in tosort:
                i+=1
                o.write(str(i))
                o.write('\t')
                o.write(line[7])        
except Exception as e:
    print e
    print "line =", line
    raise 

我收到此错误：

ValueError: time data '' does not match format '%B %d, %Y'

在此行上有日期August 3, 2001（我打印了发生错误的行）。此日期的格式正确。为什么我收到此错误？