SelectedValue which is invalid because it does not exist in the list of items

To populate drop down lists I bind the list to a lookup table in the database (i.e. States). Occasional the parent table (i.e. Customers) has a value that is “not on the list” and this error is genreated:    ‘ddlState’ has a SelectedValue which is invalid because it does not exist in the list of items. […]