TS17900102 for Edit form again, give short dump CX_HRBAS_INVALID_PARAMETER

hi,

I have created simple workflow for forms

there is approval step, with task TS17900100. this working properly

there is back to author function here, so when i check PROCSTAT after the approval workitem, in the branch from back status I have put task TS17900102

i have put below binding and also the exit class CL_HRASR00_POBJ_WF_EXIT

this is generating workitem properly in the initiator inbox then the approves clicks Backto Author option

but when I execute this workitem using HRASR_DT, I click on Send button there, I get the short dump CL_HRASR00_POBJ_WF_EXIT.

Category               ABAP Programming Error

Runtime Errors         UNCAUGHT_EXCEPTION

Except.                CX_HRBAS_INVALID_PARAMETER

ABAP Program           SAPLHRASR00_WF_UTILITIES

Application Component  PA-AS

is there something missing in binding or any other place?

thanks

bhakti