problem with a roles listbox

 i filled a list box with all the roles a have in my application, when i want to try assigning the rol to a new user i just created i get an exception. below  i put the code i have, thanks a lot for any help
 ****************** fill the listBox ******************

        lstRoles.DataSource = Roles.GetAllRoles()
        lstRoles.DataBind()
 
*************** assign the rol to the user **********
                    Dim rol As String = lstRoles.SelectedItem.Text
                    For i = 0 To (lstRoles.Items.Count - 1)
                        I ...