title: The field 'xxx' must be a number in MVC DropDownList
body: <p>I use a <code>DropDownList</code> in my MVC application. There is a problem when I load my page: there is a error message shown in the HTML source of the <code>DropDownList</code>.</p>
  
  <p>Please see this image:</p>
  
  <p><img src="https://i.stack.imgur.com/pAzBd.png" alt="enter image description here"></p>
  
  <p>This field is already set to allow null in the DB and </p>
  
  <pre><code>using System.Web;
  using Nop.Web.Framework;
  using Nop.Web.Framework.Mvc;
  using Nop.Web.Validators.Customer;
  
  namespace Nop.Web.Models.Customer
  {
   public class FileUploadModel : BaseNopEntityModel
   {
      public CustomerNavigationModel NavigationModel { get; set; }
      public string ControlID { get; set; }
      public int? EventID { get; set; }
      public IEnumerable&lt;System.Web.Mvc.SelectListItem&gt; Events { get; set; }
      public string SpeakerFile { get; set; }
      public string FirmFile { get; set; }
      public string LogoFile { get; set; }
      public string PresentFile { get; set; }
      public string Present2File { get; set; }
  
      public string SpeakerFileUrl { get; set; }
      public string FirmFileUrl { get; set; }
      public string LogoFileUrl { get; set; }
      public string PresentFileUrl { get; set; }
      public string Present2FileUrl { get; set; }
  
      public string SpeakerSubmit { get; set; }
      public string FirmSubmit { get; set; }
      public string LogoSubmit { get; set; }
      public string PresentSubmit { get; set; }
      public string Present2Submit { get; set; }
   }
  }
  </code></pre>
  
  <p>in model. </p>