How to retrieve existing value in cascading dropdown

Question

hi....
I have two dropdownlist control, one is floor & second is Shop.
Using Cascading dropdown, the data has been inserted properly but the problem is that when i click on edit button of gridview to retrieve record of particular shop ie floor name in 'cboFloor' dropdownlist & shop name in 'cboShop' dropdownlist, then the dropdownlist show error -- 'cboFloor' has a SelectedValue which is invalid because it does not exist in the list of items.
Please solve my problem how to retrive value in dropdown using ajax controls.
thanks