Project task Assignee sublist

Hi,

I am trying to load a project task and populate assignees but it is giving me error saying I am trying to edit a static sublist.

I have tried both ways of submitting the assignees-

1) Load project task record and add line item to assignee
2) Select current line item and then commit the line item

None of them is working. Can someone please explain why and where am I doing the mistake.

  nlapiSelectNewLineItem('assignee');   nlapiSetCurrentLineItemValue('assignee','resource',43345, true);    nlapiSetCurrentLineItemValue('assignee','estimatedwork',0,true);   nlapiSetCurrentLineItemValue('assignee','unitcost',0.00,true);    nlapiCommitLineItem('assignee'); 

                var current_record = nlapiLoadRecord('projecttask',31053);    current_record.setLineItemValue( 'assignee','resource', 43345, 2 );   current_record.setLineItemValue('assignee','estimatedwork',0,2);   current_record.setLineItemValue('assignee','unitcost',0.00,2);                    nlapiSubmitRecord(current_record, true);