How to update a standard field when saving a sales order?

I have the following scenario:

I must update field VBKD-VSART when the sales order is being saved. I am using include MV45AFZZ.

If I write the following code in Forms USEREXIT_SAVE_DOCUMENT, USEREXIT_SAVE_DOCUMENT_PREPARE or USEREXIT_MOVE_FIELD_TO_VBKD, it will lead to a short dump (Error POSTING_ILLEGAL_STATEMENT):

LOOP AT xvbkd ASSIGNING FIELD-SYMBOL(<ls_vbkd>).
    <ls_vbkd>-vsart = '01'.
  ENDLOOP.

Form USEREXIT_MOVE_FIELD_TO_VBKD also does not work because action needs to be triggered only at saving the order.

Only way I managed to avoid the short dump is by writing the code in a enhancement in form USEREXIT_SAVE_DOCUMENT_PREPARE, but the new value is not being saved!

In the same enhancement XVBAP fields are being successfully changed, but not my XVBKD field.

What should I do different?

Solution

The update indicator was initial.

Since the order quantity was changed, the update indicator was set automatically for XVBAP, but for XVBKD it had to be set manually:

 LOOP AT xvbkd ASSIGNING FIELD-SYMBOL(<ls_vbkd>).

    <ls_vbkd>-vsart = '01'.

    IF <ls_vbkd>-updkz NE 'D' AND <ls_vbkd>-updkz NE 'I'.
      <ls_vbkd>-updkz = 'U'.
    ENDIF.

ENDLOOP.