[Code=USER_ERROR] Please enter value(s) for: Adjust Qty. By

I am using web service to add an InventoryAdjustment record, but it always throws an error as the title describes. Anyone has met this kind of problem before?

//set the item list which is to be adjusted
  InventoryAdjustmentInventoryList lists = new InventoryAdjustmentInventoryList();
  InventoryAdjustmentInventory inventories[] = new InventoryAdjustmentInventory[1];
  InventoryAdjustmentInventory inven1 = new InventoryAdjustmentInventory();
// InventoryAdjustmentInventory inven2 = new InventoryAdjustmentInventory();
 
  String itemID = "1043";
  RecordRef item = new RecordRef();
  item.setInternalId( itemID );
  item.setType( RecordType.inventoryItem );
  inven1.setItem( item );
  inven1.setLocation( location );
  inven1.setAdjustQtyBy( new Double( 1 ) );
  inven1.setUnitCost( new Double( 5 ) );
inventories[0] = inven1;
// inventories[1] = inven2;
  lists.setInventory( inventories );
this.adjustment.setInventoryList( lists );

you see I have already set the AdjustQtyBy field value. It is really strange.