ORA-01861: literal does not match format string

I am getting error ORA-01861: literal does not match format string while running below query.

If I comment last condition then I am getting 23 records (invoices).if I pass the invoice_num with last condition then I don't get error.

Please suggest how to eliminate this error.

select aia2.invoice_num, aia2.attribute6, aia2.invoice_date

from ap_invoices_all aia

,ap_invoices_all aia2

,ap_invoice_lines ail

,ap_invoice_payments aip

where aia.invoice_id = aip.invoice_id(+)

AND aia.invoice_id = ail.invoice_id

AND aia.source = 'GLOBAL_INTERCOMPANY'

AND NVL (aia.attribute1, aia.source) <> 'CONCUR EXPENSES'