HI Keith
if you want to append a new segment why do you have modify at an index which already has some values.
anyways in case you want to remive this error do like this
ls_idoc_data-sdata =st_e1bpad1vl.
In case a new segment has to be inserted then you will need to first find the place to insert at right position. please do understand that in WE30 we have a hierarchy of segment which shall always remain consistant
nabheet