table: move row up

I use vaadin 7
I have a table with jpacontainer
I want to move up a row of the table

itemId1 is the ID of the selected Item
itemId2 ist the ID of the previous Item

final int itemId1Index = container.indexOfId(itemId1);
final int itemId2Index = container.indexOfId(itemId2);

Item item1 = container.getItem(itemId1);
Item item2 = container.getItem(itemId2);

container.addItemAt(itemId2Index, item1);
container.addItemAt(itemId1Index, item2);

i get an error at:

container.addItemAt(itemId2Index, item1); why is it so not possible?