Getting entitytype 'selectlistitem' has no key defined. Define the key for this entitytype error

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MVC4

I'm getting following error.

EntityType 'SelectListItem' has no key defined. Define the key for this EntityType.

My model is as follows :

[Table("DemoTesting")]
    public class DemoTesting
    {
        [Key]
        public int ID { get; set; }
        public string City { get; set; }

        public List<SelectListItem> lstCity { get; set; }

        public DemoTesting()
        { }

        public DemoTesting(List<SelectListItem> lst)
        {
            this.lstCity = lst;
        }

    }

Controller code is:

public ActionResult DemoDropDown()
        {
            List<SelectListItem> lstmenu = new List<SelectListItem>();
            var city = (from items in ContextObj.demoContextObj
                        select items).ToList();
            foreach (var item in city)
            {
                lstmenu.Add(new SelectListItem { Text = item.City.ToString(), Value = item.ID.ToString() });
            
            }
            DemoTesting obj = new DemoTesting(lstmenu);
            return View(obj);
        }

Im getting this error when trying to get cites by LINQ.Please help

What I have tried:

As per others site i'm already added [Key] attribute in my model but still getting this error.