Make Dropdown Optional Field in MVC C# Web Application

I have an MVC C# Web Application. In a modal, I am requesting the user to enter a few fields about their company. Several of the inputs are dropdowns populated by a database. My issue is that the dropdown fields are not required, and the page is not valid unless there is a selected value. How can I make this optional? The specific error that I get regarding the "State" dropdown is: "Key:State_ID Error". If I make a selection in the "State" dropdown, page is valid and data is inserted into the database.

Controller Snippet -------------------
        public ActionResult Client()
            ViewData["Success"] = (string)TempData["Success"] ?? "";
            VM_Client vmClient = new VM_Client
                Clients = clientDao.GetClients()
            return View("~/Views/Maintenance/Master/Client.cshtml", vmClient);

        public ActionResult AddClient()
            VM_AddClient addClient = new VM_AddClient
                States = stateDao.GetStates(),
                Countries = countryDao.GetCountries(),
                TimeZones = timeZoneDao.GetTimeZones(),
                Industries = industryDao.GetIndustries(),
                CompanyTypes = companyTypeDao.GetCompanyTypes()
            return PartialView("~/Views/Shared/Modals/Client/_AddClientPartial.cshtml", addClient);

        public ActionResult AddClientPost(Client client)
            if (ModelState.IsValid)
                client.Created_By = Request.Cookies.Get("User_ID").Value;
                client.Updated_By = Request.Cookies.Get("User_ID").Value;
                    TempData["Success"] = "true";
                catch (Exception e)
                    TempData["Success"] = "false";
            var errors2 = ModelState
                .Where(x => x.Value.Errors.Count > 0)
                .Select(x => new { x.Key, x.Value.Errors })
            TempData["Success"] = "false";
            return RedirectToAction("Client", "Maintenance");

View Snippet -------------------------
                <div class="row mg-t-20">
                    <label class="col-sm-5 form-control-label">State: </label>
                    <div class="col-sm-6 mg-t-10 mg-sm-t-0">
                        <select name="State_ID" id="add_State_ID" class="form-control">
                            <option value="">Select a State</option>
                                foreach (State state in Model.States)
                                    <option value="@state.State_ID">(@state.State_Abr) @state.State_Name</option>
                    <label class="col-sm-1 form-control-label"><span class="tx-danger"></span></label>

Dapper Snippet -------------------------
        public List<Client> GetClients()
            List<Client> allClients = new List<Client>();
            string sqlQuery = "EXEC dbo.[getClients]";
            allClients = (List<Client>)Query<Client>(sqlQuery);
            return allClients;

        public void InsertClient(Client client)
            string sqlQuery = @"EXEC dbo.[insClient]
                                @Client_Number = '',
                                @Client_Initials = @Client_Initials,
                                @Client_Name = @Client_Name,
                                @Address_1 = @Address_1,
                                @Address_2 = @Address_2,
                                @City = @City,
                                @State_ID = @State_ID,
                                @Zip = @Zip,
                                @Zip4 = @Zip4,
                                @Country_ID = @Country_ID,
                                @Phone = @Phone,
                                @Fax = @Fax,
                                @Web_Address = @Web_Address,
                                @Active = @Active,
                                @Company_Type_ID = @Company_Type_ID,
                                @Time_Zone_ID = @Time_Zone_ID,
                                @Industry_ID = @Industry_ID,
                                @Created_By = @Created_By,
                                @Updated_By = @Updated_By";
            Execute(sqlQuery, client);

Model Snippet -------------------------
    public class Client
        public string   Client_ID { get; set; }
        public string   Client_Number { get; set; }
        public string   Client_Initials { get; set; }
        public string   Client_Name { get; set; }
        public string   Address_1 { get; set; }
        public string   Address_2 { get; set; }
        public string   City { get; set; }
        public int     State_ID { get; set; }
        public string   State_Name { get; set; }
        public string   State_Abr { get; set; }
        public string   Zip { get; set; }
        public string   Zip4 { get; set; }
        public int      Country_ID { get; set; }
        public string   Country_Name { get; set; }
        public string   Phone { get; set; }
        public string   Fax { get; set; }
        public string   Web_Address { get; set; }
        public bool     Active { get; set; }
        public int      Company_Type_ID { get; set; }
        public string   Company_Type_Name { get; set; }
        public int      Time_Zone_ID { get; set; }
        public string   Time_Zone_Description { get; set; }
        public int      Industry_ID { get; set; }
        public string   Industry_Name { get; set; }
        public DateTime Created { get; set; }
        public string   Created_By { get; set; }
        public string   Created_By_Name { get; set; }
        public DateTime Updated { get; set; }
        public string   Updated_By { get; set; }
        public string   Updated_By_Name { get; set; }
Stacey FontenotAsked:
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Dorababu MSenior Software EngineerCommented:
To your list item that you are binding to drop down set a default null as follows,

yourmodel.Add(new SelectListItem() {Text="None", Value=null});

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