:Mvc.Net-Child Collection found null if collection count is 1, if count >1 data Found

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.NET

MVC

model

controller

Description: When i post form with two count of puchaseordertax, i will found that but if i post with only one record (Count) it did'nt![Note:Uid is working s index and Correctly! ]
My Model:
class PurchaseOrder
{
int PurchaseOrderID {get;set;}
datetime PurchaseOrderDate {get;set;}
public virtual ICollection<puchaseordertax> PuchaseOrderTax { get; set; }
}

My View:
@model PurchaseOrder

@Html.LabelFor(model => model.PurchaseOrder.PurchaseOrderDate)
                                  @Html.TextBoxFor(model => model.PurchaseOrder.PurchaseOrderDate, "", new { @class = "form-control" })

<table class="table table-bordered">
                                      <thead>
                                          <tr>

                                              <th>
                                                 Tax
                                              </th>

                                              <th>
                                                  OtherCharge                                               </th>
                                              <th width="60px">Action</th>
                                          </tr>
                                      </thead>
                                      <tbody id="ContainerTax">


                                          {
                                              foreach (var itmTax in Model.PurchaseOrder.PuchaseOrderTax)
                                              {
                                                  Html.RenderPartial("~/Views/PartialView/Purchase/PurchaseTax.cshtml", itmTax);
                                              }
                                          }

                                      </tbody>
                                  </table>

-----------------------
PartialView [here: Model.Uid = Work as Index which start from 0 to 1,2 3 and so on ]

<tr>
 <td>
            <input  id="PurchaseOrder.PuchaseOrderTax_@(Model.Uid)__Tax" name="PurchaseOrder.PuchaseOrderTax[@Model.Uid].Tax" type="text">
</td>
<input  id="PurchaseOrder.PuchaseOrderTax_@(Model.Uid)__OtherCharge" name="PurchaseOrder.PuchaseOrderTax[@Model.Uid].T\OtherCharge" type="text"></td>

-----------------------------
So When i Submit form with two Tax, I Found value but if only one then i didn't.
My Controller:

public ActionResult Purchaseform(int id = 0)
       {
 PurchaseOrder Purchase = new PurchaseOrder();
PurchaseOrder.PuchaseOrderTax = new ObservableCollection<PuchaseOrderTax>() { new PuchaseOrderTax()} 
 return View(Purchase);
}

[AcceptVerbs(HttpVerbs.Post)]
       public ActionResult Draft(PurchaseOrder model)
       {
model.PuchaseOrderTax = No Record if Collection is <1
}

---------------------
and also Take a look at on partial View how i add new Row.

[HttpGet]
       public ActionResult AddNewRow(int finUid)
       {
           
               return View("~/Views/PartialView/Purchase/PurchaseTax.cshtml", new PuchaseOrderTax() { Uid = Uid++});
       }