How to get Sum of column in relation table , Linq, asp.net mvc 5

Question

I have 2 tables , Debtors and Payments, and i need to get for every record in debtors get sum from Payments of pay_amount column, here is i tryied to get sum of all column , but it gives me an error

The model element passed to the dictionary has the type "System.Collections.Generic.List1 [System.String]", but for this dictionary a model element of the type "System.Collections.Generic.IEnumerable1 [Sb.Models.Debtor ] "

public class Debtor
    {
        public int Id { get; set; }


    [Display(Name = "Person")]
    public int person_id { get; set; }

    [ForeignKey("person_id")]
    public Person person {get; set;}

    [Display(Name = "amount")]
    public double amount { get; set; }

    [Display(Name = "Project")]
    public int project_id { get; set; }
    [ForeignKey("project_id")]
    public Project project { get; set; }

    [Display(Name = "actions")]
    public string actions { get; set; }

    [Display(Name = "note")]
    public string note { get; set; }

    public ICollection<Payment> Payments { get; set; }

    public Debtor()
    {
        Payments = new List<Payment>();
    }
}


public class Payment
    {
    public int Id { get; set; }

    [Display(Name ="debtor")]
    public int? Debtor_id { get; set; }

    [ForeignKey("Debtor_id")]
    public Debtor debtor { get; set; }

    [Required]
    [Display(Name ="pay_amount")]
    public double Pay_amount { get; set; }

    [Required]
    [DataType(DataType.Date)]
    [DisplayFormat(DataFormatString = "{0:dd'/'MM'/'yyyy}", ApplyFormatInEditMode = true)]
    [Display(Name = "Date_of_amount")]
    public DateTime Date  { get; set; }
}




   [HttpGet]
        public ActionResult Index()
        {
            SelectList persons = new SelectList(db.Persons, "Id", "FirstName");
            ViewBag.persons = persons;

            SelectList projects = new SelectList(db.Project, "Id", "Name");
            ViewBag.projects = projects;

            return View(db.Debtors
                .Include(p => p.person)
                .Include(p => p.project)
                .Include(p => p.Payments)
                .Select( s => s.Payments.Sum( x => x.Pay_amount).ToString())
                .ToList());
        }

and View Code

@model IEnumerable<Sb.Models.Debtor>


@{
    ViewBag.Title = "Index";
}

<h2>Index</h2>

<p>
    @Html.ActionLink("Create New", "Create")
</p>
<table class="table table-bordered table-striped">
    <tr>
        <th>
            @Html.DisplayNameFor(model => model.person_id)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.amount)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.project_id)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.actions)
        </th>
        <th>
            @Html.DisplayNameFor(model => model.note)
        </th>
        <th>Pay amount</th>
        <th>Date</th>
        <th></th>
    </tr>

@foreach (var item in Model) {
    <tr>
        <td>
            @Html.DisplayFor(modelItem => item.person.Firstname)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.amount)  
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.project.Name)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.actions)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.note)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.Payments.FirstOrDefault().Pay_amount)
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.Payments.FirstOrDefault().Date)
        </td>
        <td>
            @*@Html.DisplayFor(modelItem => item.)*@
        </td>
        <td>
            @Html.ActionLink("Edit", "Edit", new { id=item.Id }) |
            @Html.ActionLink("Details", "Details", new { id=item.Id }) |
            @Html.ActionLink("Delete", "Delete", new { id=item.Id }) |
            @Html.ActionLink("Pay", "PaymentCreate", new { id = item.Id }, new { @class = "pay"})
        </td>
    </tr>
}

</table>

</script>