simple INDEX vs range scan

 I am trying to get rid of range scan - but Oracle does (as trace shows) INDEX   MODE: ANALYZED (RANGE SCAN) OF
                      'AP_INVOICES_N4' (INDEX)  when scanning ap_invoices_all - I put INDEX hint as follows:

    SELECT /*+ INDEX(aia AP_INVOICES_N4) */

I have WHERE aia.invoice_amount = v_invoice_amount  ... as a filter

But  Oracle still scanning by range  'AP_INVOICES_N4'

What do I do...