I have a data frame like this,

```
    ID   Cus_ID cl_id
0   5.0  200    0
1   NaN  200    0
2   NaN  200    1
3   14.0 200    2
4   15.0 200    2
5   16.0 200    2
6   NaN  200    3

```

From dataframe above, I am want to extract rows 0 through 4 and check if there is any values in 'ID' column has a `NaN` values. I tried this,

```
rows_needed = [0,1,2,3,4]

df.iloc[rows_needed,0].isnull().index.tolist()

```

But I get the following,

```
[0, 1, 2, 3, 4]

```

I am expecting to get the indexes of `[1,2]`. How can I get my desired output? 

When I do this,

```
df.iloc[rows_needed,0].isnull()

```

I get,

```
0    False
1     True
2     True
3    False
4    False
Name: ID, dtype: bool

```

Not sure where I am making the mistake not to get my output.