Strange error when setting values in a pandas DataFrame

Question

This loop is working to change other columns in my DataFrame. But I keep running into this error on this specific column/dataframe. I'm basically trying do in a VLOOKUP between tables in Pandas.

KeyError: 'cannot use a single bool to index into setitem'

This code is working fine with no errors. It returns the appropiate value to the "Adjusted Agency" column

for i in range(len(df_ult_cust)):

    df.loc[df.Ult_Cust == df_ult_cust['Ult_Cust'].iloc[i], 'Adjusted Agency'] = df_ult_cust['ACRONYM'].iloc[i]

But when I use this block of code, I get that strange key error listed above.

for j in range(len(df_pca_cust)):

    df.loc[df.div == df_pca_cust['pca'].iloc[j], 'Division'] = df_pca_cust['div_acronym'].iloc[j]

Note: I chose not to use the itterows Pandas function here as for some reason it was causing the code to run slower.