I like to fill the null values in a column with a formel based on other columns:

```
data['datacqtr'].fillna(data['datadate'].dt.year.apply(str) + str('Q')+data['datadate'].dt.quarter.astype(str))

```

Can you see where the problem is with my Code? Because there are some nulll values after this code. 

My Dataframe:

> 
> datadate | datacqtr 
>  2002-02-28|2002Q1
> 
> 
> 

Dataacqtr stands for the Quarter.
And sometimes I have datadate but no datacqtr (Quarter). So I like to fill up the colmn datacqtr based on the colmn datadate.