Calculated column not giving values

Content

I'm creating an AP dashboard where I want to combine the subject areas for AP invoices and payments to show in a single column whether an invoice is paid/unpaid and the payment is cleared/negotiable.

The logic should be as follows:
IF Payment Status is null, THEN Invoice Payment status, ELSE Payment Status

My OTBI code is as follows:
CASE WHEN "Payables Payments - Payment History Real Time"."- Payment Information"."Check Status lookup Code" is null THEN "Payables Invoices - Transactions Real Time"."- Reference Information"."Payment Status Name" ELSE "Payables Payments - Payment History Real Time"."- Payment Information"."Payment Status" END