telerik radcombobox control problem in asp.net

,
I have a problem in Rad comobox,

I fill radcombobox based on another drop down event,for default  value like "--select--" showing in radcombobox,appenddatabound property used true.
I have faced problem when fired first dropdown each & every time radcombobox filled with previous data.I have used
 if (cmbProduct.Items.Count > 0)
            {
                cmbProduct.Items.Clear();
            }
            cmbProduct.Items.Insert(0, new RadComboBoxItem("-Select-", string.Empty));

          cmbProduct.DataTextField = "ProductName";
          cmbProduct.DataValueField = "product_id";
          cmbProduct.DataSource = sdsProduct;//-----datasource
          cmbProduct.DataBind();//---here i got problem parameter out of range.

Please help how can i over come this problem.
Best Regards,
Mahmud