c# - RadComboBox Binding Selected Value -

i have radcombobox , i'm trying bind selected value data i'm pulling table in database.

  <telerik:radcombobox id="cborole" runat="server"  datavaluefield="usertype.id" datatextfield="usertype.value" text='<%# bind("usertype") %>' datasourceid="odscertifications"  >                                      </telerik:radcombobox> 

here data

   public partial class certification {     public certification()     {         this.courses = new hashset<certificationcourse>();     }      public int id { get; set; }     public string title { get; set; }     public string description { get; set; }     public int duration { get; set; }     public nullable<system.datetime> expiration { get; set; }     public bool isactive { get; set; }     public string createdby { get; set; }     public system.datetime createdon { get; set; }     public string updatedby { get; set; }     public nullable<system.datetime> updatedon { get; set; }     public nullable<int> usertypeid { get; set; }      public virtual icollection<certificationcourse> courses { get; set; }     public virtual systemcode usertype { get; set; } } 

the usertype has attributes value , id upon i'm trying bind datatextfields , datavaluefields. however, i'm getting javascript exceptions when trying bind way. javascript message if omitted can't see exact error message is.

here chrome js debugger telling me happening:

uncaught sys.webforms.pagerequestmanagerservererrorexception:  sys.webforms.pagerequestmanagerservererrorexception: object of type system.data.entity.dynamicproxies.certification_d985d293565cb0bfd87fa8f8f44d70acf4130d6e138a82d6e486544c53d7fe10 ...<omitted>...y.  <asp:objectdatasource runat="server" id="odscertifications" selectmethod="getactivecertifications" typename="sitefinitywebapp.controllers.certificationcontroller"></asp:objectdatasource>       public list<certification> getactivecertifications()     {         using (var db = new entities())         {             try             {                  return db.certifications.include("usertype").where(x => x.isactive == true).orderby(x => x.title).tolist();              }              catch (exception ex)             {                 exceptionmanager.logexception(this, ex);                 return null;             }         }     } 

i don't have knowledge in rad version of telerik, think problem comes fact trying bind systemcode object combobox, , can handles simple types int / string.
guess should binding usertypeid instead, i'm not sure work because it's nullable.