HSDL Key for Bank Accounts

I have created a HSDL template with below as the key for bank accounts:
Account Number, Account Type, Bank Branch Name, Bank Name, Country Code

"Currency" has not been included in the key while configuration. When I run the HSDL to update an existing bank account with currency field, it is creating a new bank account instead of updating the existing.
It seems like "Currency" as Null is being used by the HSDL even if it is not supplied.

I have a requirement to update bank accounts using HSDL. For some employees, the bank accounts have currency code and for some other, it is not present. I am unable to create a HSDL which will help for both data.