得到错误信息太多的索引器使用Pandas。代码如下

import pandas as pd
userlist = ['Smith', 'Clark']
df = pd.read_csv("sample.csv")  

for i, df in df.iterrows():    
    if (df['Last Name']) in userlist:
        df.at[1,'Runner']='Yes'        
    else:
        pass

csv:
Last Name,Frist Name,email,Runner,Vistor
Doe,John,jdoe@123.com,Yes,Yes
Smith,Shawn,ssmith@test.org,No,Yes
Brown,Beth,bbrown@school.org,Yes,No
Clark,Tom,tclark@work.com,No,Yes

我有一个大型数据集，其中包含一个需要更新单元格的名称列表，并将遍历该列表以更改单元格。请帮助。谢谢