Error AttributeError: ("'str' object has no attribute 'isnull'", 'occurred at index 0')

Question

I am trying to run an apply lambda function on an object column. How can I check that row 3 below is a Nan value

Example test scores:

100,10,Nan,50,10,Nan

def add_grade(row):
    if row[test_scores].isnull():
       return 'F'
    else:
       return 'Passing'

when I run the above I get this error: "AttributeError: ("'str' object has no attribute 'isnull'", 'occurred at index 0') "