calculations for subtotal don't work on a "new item inline"

Hi,

1. Go to Add a new invoice.
2. In the items inlines you can add 1,2,3 in the amount boxes and the subtotal will calculate the subtotal to 6 for example.
3. Now click "Add another Item".
4. Add a value into the amount textbox of the new Item inline.
5. The subtotal doesn't change.