Update Product Table with subtable. Razor code and update code of controller is working fine but [HTTPPOST] code is not working, giving errors - asp.net-mvc-4

Question

#foreach (var sprd in ViewBag.subProduct)
{
<div class="form-row control-group row-fluid">
<div class="controls span2">
<select id="text" style="visibility: hidden;"></select>
</div>
<div class="controls span2">
#sprd.subproduct_pricetitle
#Html.Hidden("title1", (string)#sprd.subproduct_pricetitle)
</div>
#Html.Hidden("subid", (int)#sprd.subproduct_id)
<div class="controls span2">
<div class="input-prepend input-append row-fluid">
<span class="add-on prepend ">£ </span>
#Html.TextBox("combined-input1", (decimal)#sprd.subproduct_price, new { #class = "row-fluid" })
</div>
</div>
</div>
}
This is a code for displaying the data on view page.
public ActionResult Update(int id)
{
IEnumerable<producttype_master> Product_Typee;
int producttypeid = Convert.ToInt32(context.product_master.Find(id).product_type.ToString());
Product_Typee = context.producttype_master.ToList();
ViewBag.productType = new SelectList(Product_Typee, "producttype_id", "producttype_name", producttypeid);
IEnumerable<product_master> products;
products = context.product_master.ToList();
//List<subproduct_master> sprdList = context.subproduct_master.Where(m => m.product_id == id).ToList();
IEnumerable<subproduct_master> sprdList = context.subproduct_master.Where(m => m.product_id == id).ToList();
ViewBag.subProduct = sprdList;
return View(context.product_master.Find(id));
}
This update code is not working well, giving error.
[HttpPost]
public ActionResult Update(product_master prdmaster, subproduct_master subprdmaster, FormCollection formCollection)
{
product_master prd = context.product_master.Where(m => m.product_id == prdmaster.product_id).FirstOrDefault();
if (prd != null)
{
prd.product_type = int.Parse(formCollection["PrdType"]);
prd.product_updateby = 1;
prd.product_updateon = DateTime.Now;
prd.product_isactive = "1";
subproduct_master sprdList = context.subproduct_master.Where(m => m.product_id == prd.product_id).FirstOrDefault();
if (sprdList != null)
{
//string latestId = context.product_master.Max(p => p.product_id).ToString();
for (int i = 1; i <= 7; i++)
{
subproduct_master sprdList2 = context.subproduct_master.Where(m => m.subproduct_id == sprdList.subproduct_id).FirstOrDefault();
sprdList2.subproduct_price = decimal.Parse(formCollection["combined-input" + i]);
sprdList2.subproduct_pricevalidity = Convert.ToDateTime(formCollection["datepicker1"]);
sprdList2.subproduct_updatedon = DateTime.Now;
sprdList2.subproduct_updatedby = 1;
sprdList2.subproduct_isactive = "1";
context.Entry(sprdList2).CurrentValues.SetValues(sprdList);
//context.SaveChanges();
sprdList2.subproduct_id++;
}
}
context.Entry(prd).CurrentValues.SetValues(prdmaster);
context.SaveChanges();
}
return View(prdmaster);
}