.Net\AJAX Listbox and updatepanel problem

I have a listbox(l1) in an updatePanel that gets populated on changing a drop-down which is in the updatepannel as well. I have another listbox(l2) in the updatepanel which can populate l1 via javascript. I have 2 two items in dropdown. Item 1 has 6 items releated to it and Item 2 has none. When the page loads Item 1 is selected in the drop down by default and its releated data(the 6 items) are loaded into l1. When I select Item2 in the dropdown, l2 is empty since there is no data related Item 2. When I click save on the page, l1.Items.count() = 6 and the save routine save the 6 items releated to Item 1 to Item 2. If I update the l1 from l2 via the client side script everything save correctly. I'm sure if this is happening due to the updatepanel or if this just a problem with the listbox. Do you know what is causing this problem?

Thanks

From stackoverflow

• It is being caused by your update panel.

  It may the

  updatemode

  You might need to set up a custom trigger for your update panel.

  Good luck!