Database update fails when using edit

Question

This exception occurs when I try to edit my database using stored procedure with Entity Framework:

System.Data.Entity.Infrastructure.DbUpdateException: 'Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.'

This is my code:

    public ActionResult Edit(int? id)
    {
                if (id == null)
                {
                    return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
                }

                reg reg = db.regs.Find(id);

                if (reg == null)
                {
                    return HttpNotFound();
                }

                return View(reg);
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit(reg regs)
    {
                if (ModelState.IsValid)
                {
                    Traning_ASPEntities1 dbcontext = new Traning_ASPEntities1();
                    dbcontext.Entry(regs).State = EntityState.Modified;
                    dbcontext.SaveChanges();

                    return RedirectToAction("display");
                }

                return View(regs);
    }

reg model class:

    namespace UserReg.Models
    {
        using System;
        using System.Collections.Generic;
        using System.ComponentModel.DataAnnotations;

        public partial class reg
        {
            public int userid { get; set; }

            [Display(Name ="First Name")]
            [Required(ErrorMessage ="Please enter your first name")]
            [MaxLength(25,ErrorMessage ="should not exceed 25 characters")]
            public string fname { get; set; }

            [Display(Name ="Last Name")]
            [MaxLength(30,ErrorMessage ="Should not exceed 30 characters")]
            public string lname { get; set; }

            [Display(Name ="Age")]
            [Required(ErrorMessage = "Please enter your age")]
            [Range(18,70,ErrorMessage ="Age must be between 18 & 70")]
            public Nullable<int> age { get; set; }

            [Display(Name ="Gender")]
            [Required(ErrorMessage ="please specify your gender")]
            public string gender { get; set; }

            [Display(Name ="Email")]
            [Required(ErrorMessage ="Enter your email")]
            [EmailAddress(ErrorMessage ="Enter a valid email id")]
            public string email { get; set; }

            [Display(Name ="Phone")]
            [Required(ErrorMessage = "Please enter your Phone number")]
            [RegularExpression(@"^(?:(?:\+|0{0,2})91(\s*[\ -]\s*)?|[0]?)?[789]\d{9}|(\d[ -]?){10}\d$", ErrorMessage ="enter a valid phone number")]
            public string phone { get; set; }

            [Display(Name ="Username")]
            [Required(ErrorMessage = "Enter your username")]
            [MaxLength(25,ErrorMessage ="should not exceed 25 characters")]
            public string uname { get; set; }

            [Display(Name ="Password")]
            [Required(ErrorMessage = "please enter your password")]
            [MaxLength(15,ErrorMessage ="Must not exceed 15 characters")]
            [MinLength(4,ErrorMessage ="must contain atleast 4 characters")]
            [RegularExpression(@"^(?=.*[A-Za-z])(?=.*\d)(?=.*[@$!%*#?&])[A-Za-z\d@$!%*#?&]{4,}$",ErrorMessage = "Minimum 4 characters, at least one letter, one number and one special character")]
            [DataType(DataType.Password)]
            public string pswd { get; set; }

            [Display(Name = "Confirm Password")]
            [Required(ErrorMessage = "please re-enter your password")]
            [DataType(DataType.Password)]
            [Compare("pswd",ErrorMessage ="Passwords do not match")]
            public string cpswd { get; set; }
        }
    }

Stored procedure to update:

    CREATE PROCEDURE [dbo].[regup] 
        @userid INT,
        @fname VARCHAR(50),
        @lname VARCHAR(50),
        @age INT,
        @gender VARCHAR(1),
        @email VARCHAR(MAX),
        @phone VARCHAR(15)
    AS
    BEGIN
        --update reg set lname='asdf' where reg.userid=@userid
        UPDATE reg 
        SET fname = @fname, 
            lname = @lname,
            age = @age,
            gender = @gender,
            email = @email,
            phone = @phone 
        WHERE userid = @userid
    END