Implement in Numpy Python

Load a .csv file into a DataFrame object by using the following statement:

df_test= pd.read_csv('http://storm.cis.fordham.edu/~yli/data/testDataOne.csv')

Answer only one question:

1. Find all students who have not paid tuition, how many of them? (the result should include those unknown)

Expected Output: 21 students not paid tuition, including those unknown.

What I have so far:

import numpy as np

import pandas as pd

#from pandas import DataFrame

df = pd.read_csv('http://storm.cis.fordham.edu/~yli/data/testDataOne.csv')

df

#set the name of the index of the table as 'FIDN', which starting from 1

df['FIDN']=np.arange(1,33,1)

df.set_index("FIDN",inplace=True)

df

#Find all students who have not paid tuition, how many of them? (the result should include those unknown)

print(len(df[df.Paid == "no"]|df[df.Paid.isnull()]))

There's an error here. HELP!