dataframe.at[] adds new column instead of adding value

I have a DataFrame:

  APPLICATION     COUNT K3_Code_Sender K3_Code_Receiver TWO
0    BCCTBL_2       0.0              0              287   x
1    BDLSMF01       0.0           9996                0   x
2    BEX00003       0.0            762                0   x
3    BEX01910  711762.0            762                0   x
4    BEX02955      60.0              0              287   x
5    BEX02970   12058.0            762              387   x
6    BEX03148     179.0              0                0   x

after following iterate and if statement(row[4] = row['TWO'], its like this just to check column position)

    for index, row in df_fin_k.iterrows():
        if row['K3_Code_Sender'] != '0' and row['K3_Code_Receiver'] != '0':
            df_fin_k.at[index, row[4]] = 2
        else:
            df_fin_k.at[index, row['TWO']] = 1

i get new column instead of seting values to TWO column:

  APPLICATION     COUNT K3_Code_Sender K3_Code_Receiver TWO    x
0    BCCTBL_2       0.0              0              287   x  1.0
1    BDLSMF01       0.0           9996                0   x  1.0
2    BEX00003       0.0            762                0   x  1.0
3    BEX01910  711762.0            762                0   x  1.0
4    BEX02955      60.0              0              287   x  1.0
5    BEX02970   12058.0            762              387   x  2.0
6    BEX03148     179.0              0                0   x  1.0

How to fix this so we would have:

  APPLICATION     COUNT K3_Code_Sender K3_Code_Receiver TWO
0    BCCTBL_2       0.0              0              287   1.0
1    BDLSMF01       0.0           9996                0   1.0
2    BEX00003       0.0            762                0   1.0
3    BEX01910  711762.0            762                0   1.0
4    BEX02955      60.0              0              287   1.0
5    BEX02970   12058.0            762              387   2.0
6    BEX03148     179.0              0                0   1.0