NOT NULL is not working in calculated Column

Dear HANA Experts,

I have created a calculated column and written the IF condition as

IF("WC_RTYFL" ='SO' AND (NOT IsNULL("WC_SRPNO")) AND "WC_SOSTA" ='CLOSED',1,0).

But NOT Is Null is not working. Some of the records having the WC_SRPNO as NULL, but it is returning as value 1. Can you pls suggest how to handle NOT NULL values.

Thanks.