问为什么我从CSV文件python中读取的列长度错误

EN

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

ca1 = []
casU3 =[]
casU5 =[]
casBig = []
df = pd.read_csv('statistics.csv')
ca1 = df['1 cassette']
casU3 = df['2-3 cassettes']
casU5 = df['4-5 cassettes']
casBig = df['More then 5 cassettes']
cas1 = len(list(filter(None, ca1)))
casUntil3 = len(list(filter(None, casU3)))
casUntil5 = len(list(filter(None, casU5)))
Bigcas = len(list(filter(None, casBig)))
genomes = cas1 + casUntil3 + casUntil5 + Bigcas
height = [cas1,casUntil3,casUntil5,Bigcas]
bars = ('No.of genomes with only 1 cassette','2-3 cassettes','4-5 cassettes','more than 5 cassettes')
y_pos = np.arange(len(bars))
plt.bar(y_pos, height, tick_label = bars, width = 0.5, color=(0.1, 0.1, 0.1, 0.1),  edgecolor='blue')
plt.ylabel('Number of genomes') 
plt.title('Statistics over %s genomes with CRISPR immune system' %genomes) 
plt.savefig('./histogramCRISPR.png')
plt.show()