Sequence contains no elements (ASP.NET MVC)

Question

I try to make custom login in my web app

Here is Model :

 public class Clients 
{
    public int ID { get; set; }
    [Required]
    [Display(Name = "Email")]
    [EmailAddress]
    public string Email { get; set; }
    [Display(Name = "Ф.И.О")]
    public string UserName { get; set; }
    [Display(Name = "Должность")]
    public string Position { get; set; }
    [Required]
    [DataType(DataType.Password)]
    [Display(Name = "Пароль")]
    public string Password { get; set; }
    [Compare("Password", ErrorMessage = "Пароли не совпадают.")]
    [DataType(DataType.Password)]
    public string ConfirmPassword { get; set; }


}

I make controller like this :

 public class ClientsLoginController : Controller
{
    public ActionResult Login()
    {
        return View();
    }

    [HttpPost]
    public ActionResult Login(Clients user )
    {
        using (OurDbContext db = new OurDbContext())
        {
            var usr = db.userAccount.Single(u => u.Email == user.Email && u.Password == user.Password);
            if (usr != null)
            {
                Session["UserId"] = usr.ID.ToString();
                Session["Email"] = usr.Email.ToString();
                return RedirectToAction("Login");
            }
            else
            {
                ModelState.AddModelError("","Email or Login not correct");
            }
            return View();
        }
    }
}

Here is View :

    <h2>Login</h2>


@using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>Clients</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.Email, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Email, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Email, "", new { @class = "text-danger" })
            </div>
        </div>



        <div class="form-group">
            @Html.LabelFor(model => model.Password, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Password, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Password, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.ConfirmPassword, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.ConfirmPassword, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.ConfirmPassword, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Login" class="btn btn-default" />
            </div>
        </div>
    </div>
}

<div>
    @Html.ActionLink("Back to List", "Index")
</div>

When I fill Email and Password I have this error:

`An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code

Additional information: Sequence contains no elements`

In this row

 var usr = db.userAccount.Single(u => u.Email == user.Email && u.Password == user.Password);

How can I fix it?