Invoice Approval Foreign Currency

Summary:

Hi, we are enable invoice approval.
There is 2 BU currently active in the environment.
ABC BU functional currency is MYR.
XYZ BU functional currency is IDR.

We have invoices for ABC BU where the invoice currency is MYR.

When we initiate the invoice approval, there is error occur where:
Issue Description: The currency conversion rate is not defined for the MYR to IDR for the Corporate conversion rate type and conversion date 2023-05-01.
Resolution: Define the currency conversion rate for 2023-05-01.

Why should we define conversion rate for MYR to IDR since this invoice is MYR to MYR not related to IDR?