The property 'Claims' on type 'sampleUserTbl' is not a navigation property. The Reference and Collection methods can only be used with navigation properties. Use the Property or ComplexProperty method.

I get this error when I tried to use ASP.NET identity on an existing database.

Here's my context class :

public partial class sampleUserEntities : IdentityDbContext<sampleUserTbl>
    {
        public sampleUserEntities()
            : base("name=sampleUserEntities")
        {
        }

        public static sampleUserEntities Create()
        {
            return new sampleUserEntities();
        }
    
        public virtual DbSet<sampleUserTbl> sampleUserTbls { get; set; }
    }

Here is the class that inherits from IdentityUser :

 public partial class sampleUserTbl : IdentityUser
    {
        public string Password { get; set; }
        public Nullable<bool> User_Flag { get; set; }
        public Nullable<bool> Emp_Flag { get; set; }
        public Nullable<bool> Pat_Flag { get; set; }
    }

And here's my AccountController :

        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Login(LoginModel details, string returnUrl)
        {
            if (ModelState.IsValid)
            {
                sampleUserTbl user = await UserManager.FindAsync(details.UserName, details.Password);
                if (user == null)
                {
                    ModelState.AddModelError("", "Invalid name or password.");
                }
                else
                {
                    ClaimsIdentity claimsident = await UserManager.CreateIdentityAsync(user,      
                                                 DefaultAuthenticationTypes.ApplicationCookie);
                    AuthenticationMgr.SignOut();
                    AuthenticationMgr.SignIn(new AuthenticationProperties { IsPersistent = false }, claimsident);
                        return RedirectToLocal(returnUrl);
                }
            }

            ViewBag.returnUrl = returnUrl;
            return View(details);
        }

While here's the UserViewModel :

 public class CreateModel
        {
            [Required]
            public string UserName { get; set; }

            [Required]
            [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
            [DataType(DataType.Password)]
            [Display(Name = "Password")]
            public string Password { get; set; }

            [Required]
            [DataType(DataType.Password)]
            [Display(Name = "Confirm password")]
            [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
            public string ConfirmPassword { get; set; }
        }

        public class LoginModel
        {
            [Required]  
            public string UserName { get; set; }
            [Required]
            public string Password { get; set; }
        }

Login is successful when I am using code first. However I need to use it on the existing database. Any idea why it generates that error ? Thank you very much !