Return Messages after PRELIMINARY_POSTING_FB01

Hi,

I am using ' PRELIMINARY_POSTING_FB01' for parking a document, but this does'nt return the return messages either error /success, so I used the code as below

CALL FUNCTION'PRELIMINARY_POSTING_FB01'IN BACKGROUND TASK
              EXPORTING
                i_tcode           = l_tcode
                i_tcode_int       = l_i_tcode
               TABLES
                 t_bkpf           = i_bkpf[]
                 t_bseg           = i_bseg[]
                 t_bsec           = i_bsec[]
                 t_bset           = i_bset[]
                 t_bsez           = i_bsez[] 

if sy-subrc = 0.

MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4 INTO x_return_suc_msg-message.

*              APPEND x_return_suc_msg TO i_return_suc_msg.

endif.

But this throws an error saying Message is of type 'A' 'E' 'I' 'W' 'X'.

Please let me know how to get the messages returned here.

Regards,

Mohammed