Assignment of a NULL value to a NOT NULL column is not allowed.. SQLCODE=-407, SQLSTATE=23502

I am running a complex query from my application, a part of it is as

SELECT
...
NULL as column_A,
NULL as column_B,
...
FROM
...

But I am getting error as mentioned below, when i included the "NULL as column_A" phrase in the query, if I remove this phrase everything runs fine. 

ERROR : Assignment of a NULL value to a NOT NULL column is not allowed.. SQLCODE=-407, SQLSTATE=23502

But I need these NULL columns to be populated through some business logic in next step, and cant remove them. How to add them in the query statement ?