Problem using BAPI_PR_CHANGE - Error: Creation Indicador B invalid

Hi Experts:

I'm trying to execute BAPI_PR_CHANGE to modify a PurRqs.

My code is (from a test program):

* Step 1
s_number = '0010028817'.

  CALL FUNCTION 'BAPI_PR_GETDETAIL'
    EXPORTING
      NUMBER = s_number
    TABLES
      RETURN = ERRMSG
      PRITEM = S_ITEMEXP.


* Step 2

READ TABLE S_ITEMEXP INTO WA_ITEMEXP
       WITH KEY MATERIAL = '000000000000216255'.


  S_ITEM-PREQ_ITEM  = WA_ITEMEXP-PREQ_ITEM .
  S_ITEM-MATERIAL   = WA_ITEMEXP-MATERIAL  .
  S_ITEM-QUANTITY   = 48  .
  S_ITEM-PLANT      = WA_ITEMEXP-PLANT     .
  S_ITEM-PUR_GROUP  = WA_ITEMEXP-PUR_GROUP .
  S_ITEM-PREQ_NAME  = WA_ITEMEXP-PREQ_NAME .
  S_ITEM-SHORT_TEXT = WA_ITEMEXP-SHORT_TEXT.
  S_ITEM-SUPPL_PLNT = WA_ITEMEXP-SUPPL_PLNT.
  S_ITEM-item_cat   = '0'.

  APPEND S_ITEM.

  S_ITEMX-PREQ_ITEM  = WA_ITEMEXP-PREQ_ITEM .
  S_ITEMX-MATERIAL   = 'X'.
  S_ITEMX-QUANTITY   = 'X'.
  S_ITEMX-PLANT       = 'X'.
  S_ITEMX-PUR_GROUP   = 'X'.
  S_ITEMX-PREQ_NAME   = 'X'.
  S_ITEMX-SHORT_TEXT  = 'X'.
  S_ITEMX-SUPPL_PLNT  = 'X'.
  s_itemx-ITEM_CAT  = 'X'.

  APPEND S_ITEMX.

* Step 3:
  s_number = '0010028817'.

  CALL FUNCTION 'BAPI_PR_CHANGE'
    EXPORTING
      NUMBER    = s_number
      TESTRUN   = S_TEST
    TABLES
      RETURN    = ERRMSG
      PRITEM    = S_ITEM
      PRITEMX   = S_ITEMX
      PRITEMEXP = S_ITEMEXP.

The message is:

E MEPO 081 Creation indicator B invalid

E BAPI 003 Instance 10028817 of object type PurchaseRequisition could not be changed

Anybody can help me with this issue.

Thanks

Diego