Custom Fields not filled when parking with BAPI_ACC_DOCUMENT_POST

Dear Experts,

I've added some custom fields to BKPF table and have successfully filled the custom fields when posting using BAPI_ACC_DOCUMENT_POST. But when parking the document using the same BAPI with the extension field indicator:

  wa_extension1-field1 = 'BAPI-PARK'.

  wa_extension1-field2 = '1'.

  APPEND wa_extension1 TO it_extension1.

somehow the custom fields are not updated. How to make the custom fields updated too even when the document is parked?

Thank you,

Suwandi C.