null output ap_invoices_pkg.get_approval_status 11i

Good afternoon everyone,
I need your help regarding this function to get the invoice status.

ap_invoices_pkg.get_approval_status.

I tried to used this function in my query for my custom report in 11i but the column returns null when I run it in Oracle sql developer
I their any setup needed to make this work?

Please refer below for my sample query.

SELECT I.INVOICE_ID, I.INVOICE_NUM, APPS.AP_INVOICES_PKG.GET_APPROVAL_STATUS
            (
             I.INVOICE_ID
            ,I.INVOICE_AMOUNT
            ,I.PAYMENT_STATUS_FLAG
            ,I.INVOICE_TYPE_LOOKUP_CODE
            ) Approval_Status
FROM   AP_INVOICES_ALL I;