RadzenDropDown get error in filter

Help me please. I get error when filter. This is my Code

     public class upuCountry
            public int Id { get; set; }
            public string Code { get; set; }
            public string Desc { get; set; }
            public string RusName { get; set; }

      <RadzenDropDown AllowClear="true" AllowFiltering="true" 
        Placeholder="Select..." TValue="string" Data=@customers.AsQueryable() 
        TextProperty="Desc" ValueProperty="Code" />

@code {

    IEnumerable<upuCountry> customers;
    protected override void OnInitialized()
        customers = db.upuCountry.ToList(); 

This is error

I'm unable to reproduce such problem:

If you have Radzen subscription you can send us your project to debug it in order to provide more info.

Thank you. Your sample is work good.
May be due to data source

@page "/dropdown"
@inject ApplicationDbContext db;

Thank you. Now works